I have a WordPress blog running on the old version, v2.13.
There are some updates between them.
I upgraded it last week. It is not difficult to upgrade WordPress 2.5.1, but following steps should taken to avoid crash.
1) Backup data: backup database and export everything from WP.
2) De-active all plugins.
I have following plugins.
Akismet, ExtendedLive Archive, Get Recent Comments, Google Site Maps, Sideblog, Simple Tag, and wordpress stats.
3) Backup all physical files.
I SSH the hosting server and tar all files.
4) Upload WordPress 2.5.1
5) Update database
6) Enable plugins and update plugins.
Click here to download latest WordPress.
Click here for the official upgrade guide.