Go to your working copy and open a shell with following command:
svn update svn merge -c -<revision number> <url> svn commit --message "rolled back to revision <revision number>"
This will merge the content of the url from the specified revision to your working copy and afterwards you can commit. Please note the „-“ before the revision number!
e.g.: You are on revision 100 (HEAD) and want to go back to revision 78.
Checkout the HEAD to a working copy and go to this working copy in shell
svn update . svn merge -c -78 http://myserver/svn/myFantasticProject/xsd svn commit --message "rolled back to revision 78"
After that, revision 101 will contain the content of revision 78 of this specific url.