Things You'll Need
Instructions
Disassemble the minifig. This can be the hardest step, especially if the minifig has an astronaut or medieval helmet: The suction these pieces create will trap the head piece inside of the helmet. The trick is to separate the helmet from the head first, before you remove the head from the rest of the minifig. If the helmet and head come off together, your only recourse is to use a pair of needle-nose pliers to gently grasp the base of the head while you twist the helmet. Don't hold onto the head too firmly or you'll scratch the plastic head.
Replace the head. The face is painted on this piece, and cannot be changed---but you can replace the head. Simply replace the head with the face you don't want with a head that has the face you want, then reassemble the minifig.
Some minifig heads have two faces, making changing the face a matter of turning the piece around. The first minifig with a double-sided head was the Quirrell character, which is part of the Lego Harry Potter theme.
Consider replacing the head with a custom head. You can also modify a suitable Lego block to create the face of your choice. Lego sells blank heads in five different colors, including clear. Keep in mind that crafting a custom face requires skill and patience to design and decorate the small surface of the block---which is only 1/4 inch high.