Very late to the party here but everyone seems to have forgotten something. The very first step before you do any of the similar listed steps is to :

1. DEACTIVATE THE THE THEME TO BE CHANGED IF IT CURRENTY ACTIVE. CHOOSE A SECONDARY THEME (SHOULD ALWAYS BE THE MOST CURRENT WP 'TWENTYTWENTY*' THEME AS A FALLBACK THEME).
2. NOW CARRY OUT THE STEPS IN THE OTHER ANSWERS ABOVE.
3. REACTIVATE YOUR CHANGED-NAME THEME.

In short don't modify in-memory files!