Gently use your fingers to remove the key caps from the keyboard by firmly grasping the key and pulling directly up.
When putting a new or existing key cap back on make sure the pins line up and it is facing the correct direction. Once you have done this, firmly push it in until you hear it click into place and works like normal again.
If the key cap doesn't push in all the way or won't pop back up from being pressed down then it is either in the wrong spot or oriented the wrong way. Pull it off, make sure no debris is blocking the pin hole and try putting it the correct way again.