if your not far in honestly just reload the game. if you are doing an import ME2 then when you pick a facial structure pick option 2 customise and you can repick your class like 2 screens in from there.
can not do this on a import ME3 though as it locks down to how you played the previous play on it
--------------------------------------------------------------------------- am I staring into the Abyss or is the Abyss staring into me? maybe its just staring into it's own reflection - unknown