Is it possible to upgrade from 3.2.1 to 3.3 without losing all modifications I have made in the editor?
Let that be a lesson to you young man, child themes are your friend!
Kidding of course. :) It's (probably) not the end of the world. You only made changes to the TwentyEleven theme?
Give this a try:
1. Backup your files and database. Easy advice and always step #1.
http://codex.wordpress.org/WordPress_Backups
http://codex.wordpress.org/Backing_Up_Your_Database
http://codex.wordpress.org/Restoring_Your_Database_From_Backup
2. Once that's done, make a copy of the wp-content/themes/twentyeleven directory to something new like wp-content/themes/twentyeleven-frisyren.
3. Edit wp-content/themes/twentyeleven-frisyren/style.css and change the second line from
Theme Name: Twenty Eleven
to
Theme Name: Twenty Eleven Frisyren's Copy
Go into the http://your-blog-url/wp-admin/themes.php and select the theme copy you've just made.
You should be now using that separate copy of the theme and everything should look fine.
4. Now get the 3.3 download from http://wordpress.org/latest.zip and do a manual update.
http://codex.wordpress.org/Updating_WordPress#Manual_Update
I'm suggesting manual update because you do not want to mix and match files. The upgrade process now does incremental updates (I think, I'm an SVN addict myself so it doesn't come up for me as an issue).
If something goes catastrophically wrong then restore your files and database and you'll be back to square one.