Once you have opened a song in the playback window, if transpositions are available you will see a transpose button in the top toolbar.
In this menu you will see a list of available transpositions. The key that the music is currently transposed to will have a green check on the right side of its row. To switch to another key, click its row. If you see "download" on the right side of the row, the transposition will need to be downloaded and synched to your app before you may open it. Otherwise the transposed file already exists on your computer, and you can switch to another key without having an internet connection.
You can also transpose a piece, if transpositions are available, while viewing the preview in the main navigation window.
Please note: Some pieces currently do not available transpositions, or are arrangements that are not transposable. If this is the case, when you open the transpose menu in either the viewer window or preview, you will simply see a message saying "no transpositions available."