Press that giant "Plus" button down there
Select an instrument family
Yes, that's it. Add other instruments as you wish.
(optional) save your current session by tapping the save button at the top center of the screen (or at the bottom, if the pull-down menu is opened).
Delete all the instruments by long pressing the Mixer thumb at the bottom of the screen (if you don't see it, close the pull-down menu).
Add new instruments.
Please take a look at the Glossary or write me at possiblemusicapp@gmail.com