change factor level 1,2,3 to red,blue,dark

I am new to R. Maybe this is very simple question.
I have a dataframe, there is column that is factor.
This factor has three level that 1,2,3.
Now I want to change these  level(1,2,3) to level(red,blue,dark).
Does anybody how to do this job?

Thank you very much

Aimin