@Myrrh ...
The original poster of this thread did have Linux, so the information shared was for Linux. Am not a Windows person, but the concept is the same. From your other postings, sounds like you are remotely hosted so do it the way you've done it before (download, unzip, ftpupload, hit notifications of site), but ....
2.x is a massive change from all previous versions of Moodle. It does more and requires more. Matter of fact, some folks have found out the hard way their hosting provider cannot provide the supports needed for Moodle 2. So *before* you try this make sure your hosting provider has the minimum for PHP and MySQL. From the downloads page: 2.5.1+ Requires: PHP 5.3.3, MySQL 5.1.33.
If they don't have, seek another provider.
Either way you go, there will be a learning curve. But, that shouldn't deter an educator - it called new learning! We expect students to do this every day. Why should educators be 'excused'?
You might consider a different path. Others have chosen the following and they may have had some issues depending upon their 1.9 site/courses. If, however, you have been using a 'stock' Moodle (no addons/no special themes, etc.) then the chance for issues are less.
Backup all of your courses in the 1.9. Download those backup zips.
Install a fresh 2.5.1+ and get it up and running.
Restore the backups.
This will not bring students accounts into the new site but one can still use bulk user upload. Since no accounts, it's logical that restoring 1.9 backups will not include any student work ... ie, assignments they turned in or quiz/test they took, but the Assignments are there as well as the quizzes/test. Therefore it's probably best to do this in between online sessions with students (end of semester/grading period, etc.).
Maybe there's a Windows person out here in Moodle land that can jump in here or provide a link with info.
'spirit of sharing', Ken