How do I Upgrade Subversion to 1.6.x?

1. Download the latest Subversion 1.6.x installer. http://subversion.apache.org/packages.html
2. Log onto your subversion server.
3. Run the installer, making sure that you select the same Subversion directory as the current 1.x install. (i.e. C:\Program files\Subversion)
4. If you are running Apache, let's make doubly sure everything is using the current dll's. Stop your apache service, and copy over (overwriting) the following files from ...\Subversion\bin\ folder into:
   
   ...\Apache2.2\modules\
   mod_authz_svn.so
   mod_dav_svn.so
   
   ...\Apache2.2\bin\
   libapr-1.dll
   libapriconv-1.dll
   libaprutil-1.dll
   libeay32.dll
   ssleay32.dll
   
   
5. Restart your apache service.
6. Double check that you can still access your svn through TortoiseSVN or whatever SVN client you are using (no point continuing if something is not working. Troubleshoot your install, and when it is working, continue on).
7. Open a command prompt.
8. Run the following command to upgrade your repository to the latest version (1.6):

svnadmin upgrade [Path to Repository]

Note: You have to manually upgrade your repositories to the latest version because the repository is not upgraded automatically. See:  http://subversion.apache.org/docs/release-notes/1.6.html#repos-upgrades

If you are upgrading from a pre-1.0 Subversion environment, you may have to do a "dump and load" See:  What is this "dump/load cycle" people sometimes talk about when upgrading a Subversion server?