A track can contain one or several regions, i.e. colored rectangles displaying either a waveform or midi data. You can change the pitch of a region by clicking "Edit" on the region (1) and selecting "Change Pitch..." (2). This opens up a dialog with a pitch button that you can click and drag up or down to change the pitch in semitones (3). 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. Click "Change" when you're done. This works both for audio and midi regions. The changes will only affect the region you click on, so if you have more than one on the same track that you want to change, you will have to do it one at a time.
If you have a melodic loop containing pitched audio, like bass or guitar or piano etc., you have a Transpose function instead of the usual pitch change. This is a very cool feature! If you hover over a melodic loop, it will display the key that it is currently in (4). Click "Edit" on the region and then "Transpose" (5) to change it to a different key. Hover over the region again and it will display the new key (6). 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) Change Pitch dialog (click and drag the button up or down to change the value - 1 semitone equals one key on the piano or one fret on a guitar)
(4) Edit region (melodic loop containing pitch/key information)
(5) Edit region - Transpose
(6) Region pitch/key information