I would like to upgrade Subversion 1.5.1dfsg1-4 (from lenny) to 1.6.4dfsg-1~bpo50+1 (from lenny-backports) on my Debian 5 machine. Which steps do I need to take? Do I need to backup my repository, or can I just install the new package over the old one?
You should have backups of your repository anyways. But if not I would make one before you upgrade.
However according to the 1.6 release notes