Using the chromatic circle to count keys, change the note names in all of the chords by the same amount (the same number of half steps, or places in the chromatic circle) and in the same direction. Change only the note names (things like "F" and "C sharp" and "B flat"); don't change any other information about the chord (like major, minor, dim., 7, sus4, add11, etc.). If the bass note of the chord is written out as a note name, change that, also (using the same chromatic circle).
Check your transposition by playing it to see if it sounds right. If you don't like playing some of the chords in your new key, or if you have changed the key too much or not enough, try a different transposition.
Say you have a song in the key of G, which is too low for your voice. If it's just a little too low, you can go up two keys to A. If this is still too low, you can go up even further (5 keys
altogether) to the key of C. Maybe that's high enough for your voice, but you no longer like the chords. If that is the case, you can go up two more keys to D. Notice that, because the keys
are arranged in a circle, going up seven keys like this is the same as going down five keys.
Now say you have a song in the key of E flat. It's not hard to sing in that key, so you don't want to go far, but you really don't like playing in E flat. You can move the song up one key to
E, but you might like the chords even better if you move them down one key to D.
Notice that if you are a guitar player, and everyone else really wants to stay in E flat, you can write the chords out in D and play them with a capo on the first fret; to everyone else it will sound as if you're playing in E flat.
Now say that you have a song that is in B flat, which is more than a little (more than one key) too high for you. Find a key a bit lower that still has nice, easy-to-play chords for guitar.