- wire snips
- pliers (maybe)
- a cloth to wipe down guitar
- guitar polish (optional)
- a "string winder" (optional but recommended)
Begin by finding a flat surface on which to lay the guitar. A table works well, but the floor works in a pinch. Position yourself in front of the instrument, with the guitar's sixth string closest to you. Completely slacken the sixth (lowest) string of the guitar, by turning the tuner. If you're unsure of which direction to turn the tuner to slacken the string, pluck the string before you begin turning the tuner. The pitch of the note should get lower as you slacken the string.
Once the string has been completely slackened, uncoil it from the tuning peg at the head of the guitar. Next, remove the other end of the string from the bridge by removing the sixth string bridge pin from the bridge of the guitar. Commonly, bridge pins will provide some resistance when trying to remove them. If this is the case, use a pair of pliers and gently coax the bridge pin out of the bridge.
Discard the old string. Using your cloth, wipe down any areas of the guitar you can't reach with the sixth string on the instrument. If you have guitar polish, now is the time to use it.
It is important to note that some guitarists remove all strings from their guitar at once and then replace them. I highly advise against this procedure. The six tuned strings of a guitar produce a great deal of tension on the neck of the instrument, which is a good thing. Removing all six strings at once drastically changes this tension, which many guitar necks don't react well to. Sometimes, when all six strings are replaced, the strings will sit impossibly high off the fretboard. Change your strings one at a time to avoid a variety of issues.