how to remove a theme

Hi,
wP 2.6.5 works very well.
I’ve installed a theme called Multiflex 3 http://www.temi-wordpress.com/305/multiflex-3/ and my blog no permits more login.

Perhaps the theme is too old but ****I don’t log in to change it.*****

How I can change the theme? or where I find in database or in a config file the exact place to change it?

TIA
Roberto