To change your main instrument, go to your account by tapping the fourth icon of the tab bar.
On your account page, just tap on your instrument to see the list of available instruments again. If you change your instrument you will be asked for your level aswell.
Whenever you play a new piece, the best available accompaniment for your instrument will automatically be selected, and the score is transposed if you chose a transposition instrument.
You will also get recommendations according to your new instrument and level.