I have liftmasterPro garage door opener. How do i

If your other remote units work okay then your keypad has failed.

Chec your receiver unit located somewhere by the door usually you could change the code your self.

may need service by the expert.

You cannot change the code of a remote. You can change the code of a keypad.

Are you sure you bought the correct remote? All are not the same.

To program the keypad, first unplug the garage door opener from the electrical outlet. Next remove the nameplate from the console that the keypad plugs into and locate the program/operate switch. Flip the switch to program. Next enter your desired four-digit PIN number into the keypad. Slide the switch back to operate and plug the garage door opener back in. The keypad should be programmed.

If you reset your code by pressing the smart/learn button and then keying in your code and pressing enter and it still doesn't work it has just quit on you. Make sure if you replace it you get the same model number.

is it little buttons on the remote change the buttons on the back of your operator to the same has the ones inside your remote. or if it's a newer one you can hold the square button of the back of the operator in for 8 secs. then after that you push the button and let go and then push the button on your remote. done

If you want to change the keyless entry code just press the green learn button on the motor, Enter a four digit code and press the enter button within 30 seconds.

Follow this link for the manual and look on page for programming.
http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=oMg&q=liftmaster+garage+door+openers+manual&btnG=Search

There is a "learn" button on the garage door opener unit itself that you press before entering a code on the outside transmitter. This allows the receiver to learn a new code. The button is on the rear end of the housing near the terminals where the inside operation pushbutton connects.
Here's a link to programming instructions for a compatible unit by another manufacturer:
http://www.usdoorcontrol.com/instructions/liftmaster/keypads-66LM.pdf