Brita faucet filter adapter is stuck in my faucet

refer to instruction.
Move the filter handle toward you to activate the filtered water

take old aerator to home depot, lowe's and use their thread identifier for aerators.

you can use a vice grip needle nose pliers available at a hardware store or if you have a medium size regular one that would work also just not as good.. can't get as good a grip. or maybe a friend can loan you either one. preferably vice grip style.
it'll work where the fliers won't fit.
hope this helps a little.

The bonnet nut may be to tight.Turn off water to faucet and remove handle with allen wrench at set screw then unscrew bonnet nut to reveal bonnet washer and valve ball there are also springs and washers(take note how it is taken apart because it must go back in same way it came out)then clean all components use some plumbers grease or vaseline to coat parts when replacing it will help in the movement of handle also,(you may want to put in repair kit since you have apart,availabe at home improvement,hardware,plumbing supply stores,I do recommend this).

I have a Brita faucet filter (model FF-100) that lately has become harder and harder to turn. I toyed with the idea of forcing the unit apart but, instead opted to lube the mechanism directly from the top, where the unit attaches to the faucet.
To this mean, you will have to 1) Detach the unit from the faucet. 2) Remove the filter. 3) Thoroughly wash the unit, inside and out, turning the knob to flush any debris that may be present. 4) Pour some drops of vegetable oil inside the mounting opening, coating the valve stem within. (Oil may pass thru the unfiltered water outlet, so choose an appropriate working space). 5) Turn the water handle (knob) to lube the valve repeatedly, until it loosens. 5.b) Optionally, you could let the oil sit for some time inside the base. 6) Rinse the unit and reinstall it on faucet (without the filter). 7) Turn knob to unfiltered position, and flush with warm/hot water. 8) Do likewise with knob set to filtered, but open faucet only slightly, to avoid over-spraying. 9) Remove base unit from faucet and repeat steps 3-9, as needed. 10) Reinstall unit with filter, as instructed by manual.
Water valves may seize due to mineral deposits. While you may be tempted to use detergents or solvents, be reminded that unless thoroughly rinsed, chemicals may corrode plastics, degrade your filter, or even wind up in your drinking water, so caveat.

Before you start taking the whole faucet apart try taking the airiator or head of the end of the faucet. It may just be some sediment blocking the filter screen on the head. Unscrew the head and see if the water flows with the head off.

I see this alot when I do installs...contact the Brita tech support
1-800-24-BRITA (US)

Want a free adapter and patient enough to wait for a couple of weeks to get them?
"To receive a special adapter free of charge, call 1-800-24-BRITA or 1-800-387-6940 (CANADA)."
Call the number above and a voice prompt will give you instructions on how to get your adapter for free. 2 weeks might be too long but the best thing is it is made of stainless steel/chrome and it comes in 3 different kind of adapters with rubber washers.

Hold filter cartridge in one hand and slowly turn on water. The water pressure will push the cartridge out of the housing.

When mine gets stuck I stick the handle of a fork or spoon into the hole where the water comes out and use that to unscrew it. Works like a charm every time.