To tune a guitar one semitone flat, you simply fret each string at the first fret, and then tune the guitar with the string fretted. You can do it one string at a time, or use a capo.
Say the guitar is already properly tuned. You fret the bass E string at the first fret. The tuner hears F, and tells you you need to tune down one semitone. When you release the string, the note will be E flat instead of E.
I used to tune down all the time to play Stevie Ray Vaughan songs.