Most Nuvis have a procedure to perform a 'Master Reset', this will usually erase any Favourites and return the unit to factory default so I would normally suggest backing up anything you want to keep (if possible) before doing this. The procedure varies slightly from model to model, but step include :-
Ensuring the unit is off (possibly not valid in this case)
Pressing and holding the power button for 10 seconds should display "Turn off Device" message
From 'Off' touch and hold the lower right hand corner of the display
short press the power button while still touching lower right corner
Garmin logo appears and copyright info.
When "Erase All Data Y / N" is displayed remove 'lower corner touch'
Select Yes
On some models removing 'touch' for a short period and the re-applying is required