Guitar Tunings

That is why you can.

Right click the staff
Select Staff Properties...
Click Edit String Data... at the bottom of the new window
Click a string tuning (DO NOT CHECK THE BOX) and click Edit String...
Click the new String tuning
Click OK until you're back at your score.

In addition to tuning changes, one can modify the number of strings, even frets.

The handbook explains much. See:
https://musescore.org/en/handbook/tablature#edit-string-data

Regards.