Can I change my appearance?

You can get the Mass Effect 3 save editor here.

Start a new game to customize a face, then play until you are able to save.
Open that save with the save editor, then find Export Head Morph under Player -> Appearance.
Open your current playthrough in the save editor and import the face.

You unfortunately can't change your appearance mid-game (not without a belt-sander....), though you CAN change your imported character's appearance when you import your character. This can only be done at the start of the game.

There are some unoffical projects that are trying/planned to edit ME3 save files like this one but as of yet you can not edit Mass Effect 3 save data. Give it some time and this may change however, the game hasn't even been out a week and the save data is very complex.

There is no way to change your look once you step in game. All decisions made during character creation are permanent.

There was an unofficial save editor for Mass Effect 2 that allowed you to change your face, so if you wait a while (or search hard enough) you should be able to find something similar.