2
easiest way:
STEP 1: make a full backup from your site: files and database! I warned you.
download the latest 2.0.x stable release (at this time 2.0.14). Unpack the archive, and remove the following files from the package:
htdocs/mainfile.php
htdocs/install/ (complete folder and all files in it)
Next, upload all files from the htdocs/ folder onto your webserver, overwriting the 2.0.11 files with the ones from the package!
Then, login to your site as admin, go to system admin -> modules and 'update' the system module.
That should do the trick!
Herko