To edit an instrument, or add a new instrument to an instrument panel, long press on an individual instrument.
To edit your instruments:
Delete an individual instrument by pressing the β button on the instrument
Move an instrument by dragging it
Add Instruments by pressing the + button
Adding an instrument:
Select the category of instruments you want to add
Select the type of instrument inside the category
Swipe between the different instrument options to preview different sizes
Press Add Instrument to add it to your current panel