Have you taken some time off from playing or would just like to recap everything you've learned and start from the beginning? Now you can with the progress reset feature. Please note that once progress is reset, it can not be reversed.
Pick the instrument you want to reset progress for under Settings → Game → Instrument. Then go to the Account tab where you'll find the reset option.
Tap the Reset button and check you have chosen the instrument you wish to reset progress for. You can exit the screen by pressing the X or Cancel button.
Tapping on Reset button again will open a new screen where you can double check that the correct instrument is chosen. You can also still cancel reset by tapping on X or No button.
By tapping on Yes, the mission progress for your chosen instrument will be reset, and you can start over in the instrument syllabus.
The reset does not affect your scores played in Weekly Challenges or songs that are not included in the instrument syllabus accessible via Learn → Missions tab.