A great tutorial about branching/merging in Subversion can be found under
http://blog.evanweaver.com/2007/08/15/svn-branching-best-practices-in-practice/

Applying Diffs in Subversion

1. Creating a patch: A diff file can be created with

svn diff > patch.diff</pre>

Be aware about the path where you create the patch.

2. Applying a patch: A diff can be applied with

patch -p 0 < patch.diff

Be aware about the path where you create the patch.