If you are looking to upgrade your 1.9 tikiwiki version to 2.0 one. The released packages includes an upgrade script in tiki-install.php file, just browse it and ensure the you understand the changes in the category permissions introduced in Tikiwiki 2.0.
The CVS instructions below no longer apply. See http://dev.tiki.org/tiki-index.php?page=SVNTips for checking out 2.0 from SVN
This can generate conflicts if you made local changes, so, if it's on production code, do it on a copy.
Read Where to commit. Remember that these are not rules set in stone - they are guidelines that hopefully represent community consensus, but this is a wiki. Feel free to contibute.
(The following tools need to be updated for SVN. They were designed for CVS, which the 2.0 branch has now moved away from)
It's quite advisable to merge your own commits, because you are the most able to resolve conflicts if any occur. There is a script for that. It have been tested for years but it was mainly displaying lines that was copypasted by the merger, just to check at each steps that everything went fine. Now that we know how it flows, we made this script interactive. It assumes that you run on GNU/linux as a development platform, and have bash installed. Read the file doc/devtools/tikimerge_110to111.sh so you can know how to setup your environment to fit this script if your context is special.
cvs ci -m'[FIX] wiki: fixed icons alts' templates/tiki-showpage.php tiki-listpages.php lib/wiki/wikilib.php
./doc/devtools/tikimerge_110to111.sh templates/tiki-showpage.php tiki-listpages.php lib/wiki/wikilib.php