If the file versions in both branches share a common ancestor (therefore their contents are mostly similar) and you want to propagate the changes, you probably want to merge.
This is a tricky question, and there are many ways to do this, so you probably want to read what the SVN book on the topic has to say .
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svn co http://www.example.com/svn/branches/release
cd release
svn merge -c1234 http://www.example.com/svn/trunk
svn ci -m "Merged revision 1234 from trunk to release branch."