Answer:
If you must do it manually (rather than using a composing software) you find the number of half steps it is from the original key signature to the new key signature. You then move each note in the chord that number of half steps (down or up, depending on what you are doing) in order to find the new notes for the chord.
It takes a lot of time to do that to an entire song, but I don't know of any other way!