A track can contain one or several regions - colored rectangles displaying either a waveform or midi notes. You can change the pitch of a region by clicking "Edit" on the region (1) and selecting "Change Pitch..." (2). This allows you to change the pitch in semitones.
A semitone equals one key on the piano keyboard - if you go one semitone up you're moving one key to your right, and vice versa. This works both for audio and midi regions.
If you have added a loop from the loops library containing pitched audio, like a bass or synth loop, you have a Transpose feature instead of the usual Change Pitch. This is a very cool feature! If you hover over a loop, it will display the key that it is currently in (3). Click "Edit" on the region and then "Transpose" (4) to change it to a different key. The region will update to display the new key. You can build an entire song simply by copying regions and transposing them in this way :)
(1) Edit region
(2) Edit region - Change Pitch...
(3) Edit region (loop containing pitched audio) Note the chord information on the region (Am)
(4) Edit region - Transpose