Cant remove screw in chuck to replacechuck

The greatest one that fit most chucks are 13mm but remember to remove the torxscrew (in clockwise direction) inside the chuck first and if you use a new chuck beware of to use some loctite on the screw.
The long side in a vice the short in the chuck and turn the left anticlockwise direction the L direction and pull short but fast the trigger and move the drill in the same direction turning the drill has priority to make doing this in a very short time. to not overcharge you tool.

You don't say which drill you have, but this is one method.

Remove the left handed screw from inside the chuck, put the drill into low gear then lay the machine on it's left side, fit a suitable spanner on to the slot behind the chuck and arrange it so that the spanner is against the work surface. Put the chuck key into the chuck and give it a hard blow so you knock it in an anti clockwise direction. If it is a keyless chuck or you don't have a key try holding the short end of a large allen key in the chuck and knock the long end in a anticlockwise direction. This will work even if you don't have a spanner to fit behind the chuck but you might have to hit it harder. If you have a vise after removing the screw hold the chuck in a vice and fit the spanner behind the chuck and turn it anti clockwise.

Remove the left handed screw from inside the chuck then lay the machine on it's left side, fit a suitable spanner onto the slot behind the chuck and arrange it so that the spanner is against the work surface. Put the chuck key into the chuck and give it a hard blow so you knock it in an anti clockwise direction. If it is a keyless chuck or you don't have a key try holding the short end of a large allen key in the chuck and knock the long end in a anticlockwise direction. This will work even if you don't have a spanner to fit behind the chuck but you might have to hit it harder. If you have a vise after removing the screw hold the chuck in a vice and fit the spanner behind the chuck and turn it anti clockwise.

Set the drill to reverse. Hold the chuck, press the trigger and open the drills chuck to it wide open position. Now,, take the correct size allen wrench and insert it into the back of the chuck to remove the screw. Turn the screw clockwise to remove it. Notice I said clockwise, just the opposite of what you might think. Hold the chuck with your hand or a pair of large vise grips and press the trigger to unscrew the chuck from the drill. Rotate it counterclockwise to remove it from the drill. Reverse the procedure to install the new chuck.

Remove the REVERSE-THREAD screw at the bottom of the chuck. Set the clutch collar to the highest setting, and set the gear to the slowest setting on the transmission. Remove the power source! Get the biggest allen wrench you can fit in the end of the chuck(the shortest leg) and tighten the chuck down around it, allowing the jaws to rest and tighten on the flats of the wrench. Lay the drill on its side(chuck toward you, handle to the left, and allen wrench to the left, horizontal to the deck. Then with a hammer(soft hammer if you care about the allen wrench) quickly and deliberately hit the allen wrench in an attempt to cause the allen wrench to spin counter clockwise. Let me know if this works for you!

First of all, open the jaws of the chuck and look inside. If there is a screw in there you should be able to do this yourself. If there is no screw (sometimes torx/star head, sometimes flat/straight head) then yo uwill need to take it to a service agent/dealer as a special jig is needed to lock the spindle/chuck.
If there is a screw you need to remove it. It will be a left hand thread so turn it clockwise to remove. It will be proabbaly be tight as it will be thread locked in the factory. Once this is removed, clamp a 10mm allen key in the chuck adn steady the drill on a workbench. The actual chuck will be a normal thread so this is removed by turning it anti clockwise. It will take a sudden impact to start it off so put the gearbox in a low gear and turn the clutch to 'drill mode'. Holdong the machine steady you can hit the allen key sharply to start undoing the chuck. It may take several hits to get it going.

Place drill in vice with chuck in vice facing up . Clamp chuck lightly but with enough force to hold firmly.
put allen wrench in chuck and lightly tap down to make good fit to allen screw head.Clamp chuck jaws down onto allen wrench lightly to hold wrench steady. Take piece of pipe to end of wrench,10".
These chuck screws are tight. Hammer clockwise until screw brakes loose.Open chuck completely and remove screw.Chuck is threaded to spindle shaft . Place allen wrench with long end facing out of tool in chuck and tighten chuck securely. With hammer tap allen wrench sharply counter clockwise ( normal thread.)

I simply placed the chuck into a large vise and turned the drill counter clockwise and the chuck spun right off no problem without damaging anything!! It does have some thread lock on the chuck but just use your man strength and its as simple as turning the lid off the pickle jar! Do not waste your time or money with the removal tool...YOU DONT NEED IT!

to change the chuck you must remove the screw inside chuck by turning clockwise as it is cross threaded then place an allen key in chuck tighten and hit several times with hammer till chuck lossens

To remove the chuck on your drill follow the instructions for the type of drill chuck you have:
Instructions for removing a keyed chuck
Most reversing drills have a left hand threaded screw located inside the bottom of the chuck.
1.) If the unit has this screw place the chuck key, with the chuck facing toward you, into one of the holes on the right side of the chuck. Strike the key in a clockwise direction with a wooden mallet or similar object. This will loosen the screw inside the chuck.
2.) Open chuck jaws fully. Insert screwdriver into the bottom of the chuck to engage screw head. Remove screw by turning clockwise this is a left-hand screw.
3.) With the chuck facing toward you place the key into one of the holes on the left side of the chuck. Using a wooden mallet or similar object, strike the key sharply in a counter-clockwise direction. This will loosen the chuck so that it may be unscrewed by hand.
There is no way to stop the motor from turning.
It may require several attempts to loosen the chuck
Instructions for removing a keyless chuck
Most reversing drills have a left hand threaded screw located inside the bottom of the chuck.
1.) If the unit has this screw open chuck jaws fully. Insert screwdriver into the bottom of the chuck to engage screw head. Remove screw by turning clockwise this is a left-hand screw.
2.) Insert the short end of a 1/4" or larger hex Allen wrench into the chuck and tighten. Facing the tool strike the long end of the hex wrench sharply counter clockwise as it is a standard right hand thread. This will loosen the chuck so that it may be unscrewed by hand.
There is no way to stop the motor from turning.
It may require several attempts to loosen the chuck