In the Note Game mode click on the button that displays the name of the current guitar tuning.

 
 
 
 
 
 
 
 
Initial, there is only the Standard guitar tuning in the list. The Standard guitar tuning can not be modified or deleted.
The fist way:
1. Press and hold on the element "Standard" for opening the context menu of this item.
2. Select the option for creating a new guitar tuning.

 
 
 
 
 
 
 
 
The second way:
1. Click on the settings button.
2. Select the option for creating a new guitar tuning.

 
 
 
 
 
 
 
 
Edit and save data of the guitar tuning.

 
 
 
 
 
 
 
 
The new guitar tuning will appear in the list. Click on an element from the list to select that guitar tuning.