Wordpress: how to change your theme manually

So the other day someone told me how they changed their theme in Wordpress and it broke their site. It broke it so much that they couldn't get back in to change it back. So I talked them through manually editing their database to get their site back and it occured to me that this should be easier to find on the internet.

So here are some instructions on how to do it:

Open phpMyAdmin
choose your wordpress database.
select the wp_options table
search for and change the option_value of the following option_name's:

Change the option_value to be a valid theme (default Wordpress ships with twentyeleven or twentyten).