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.