svn cp https://source.sakaiproject.org/svn//sakai/tags/sakai-2.9.1-all/ https://source.sakaiproject.org/svn//sakai/tags/sakai-2.9.2-all/-m " SAK-23696 copied from 2.9.1-all tag, ready to mod"
Copy the files in the 2.9.1-all tag to the 2.9.2-all tag to create a baseline from which to work.
svn co --ignore-externals https://source.sakaiproject.org/svn//sakai/tags/sakai-2.9.2-all/
Checkout 2.9.2-all to get the files you need to edit locally.
On your local copy of sakai-2.9.2-all, edit the .externals file . You will need to update the versions correctly so that instead of 2.9.1, the file contents point to 2.9.2. You will also be updating the Indie versions which you can find in the POM file for the release. What you are doing is fixing versions of the tags that its pointing to and editing the various POMs.
svn propset svn:externals -F .externals .
the svn propset command is key to telling SVN to incorporate the source from the external projects. A nice explanation is on the Jasig wiki - https://wiki.jasig.org/display/~awills/Taming+uPortal+--+Multi-Project,+Multi-Environment+Builds