The tuner allows you to perform the standard tuning. You click on the button and the midi sound will be played. However, this is also the place where you can change the tuning of your virtual guitar. Click on Settings → Retune Guitar and choose in the submenu to either retune All at once or One by one. The All at once choice allow all notes (strings) to simultaneously move up or down a half note by either left-click or right-click on the tuner with you mouse buttons. One by one allows individual adjustment of each string. The last choice is a Reset that brings you back to the “untuned” state.

Note: You can only shift the tuning of the guitar up or down by 10 half steps.