Make sure you have at least "Version 3.0.3" of the app!
To reset the tuning of any string instrument to the default tuning, make sure the instrument is visible, and a string instrument is selected.
👉 Tap on the instrument title:
👉 Tap on the tuning you have selected (i.e: "Drop D", "Open A"...)
👉 In the list which opened above, scroll all the way to the left horizontally and select "Standard".