Example of merging the 1.5 release back to the trunk:
svn merge http://joomlacode.org/svn/joomla/development/trunk@HEAD http://joomlacode.org/svn/joomla/development/releases/1.5@HEAD /Applications/MAMP/htdocs/joomla/trunk
The arguments are svn merge A B C where you want to change A in B and put it in C.
You may like to use the --dry-run option first just to test the command. Merging may take some time. It is a good idea to use the command line to perform this. If you are using Eclipse you will not be able to run this in the background.
See http://svnbook.red-bean.com/en/1.1/ch04.html for an explanation of the concepts behind branching and merging.
On occasion you may get an error message like:
svn: Commit failed (details follow): svn: PUT of '/svn/joomla/.../development/trunk/libraries/joomla/application/helper.php': 423 Locked (http://joomlacode.org)
To force unlock it remotely, navigate to the correct folder and run the following command:
svn unlock --force helper.php
See http://svnbook.red-bean.com/en/1.2/svn.advanced.locking.html for more information on locking.