I want to roll back my SVN repository , I just want to tell this revision now should be a head revision .
Is it possible to do so in SVN ?
You can't officially "roll back". You can copy the favored version to a new node just off the root, and then delete everything else, and then move all the children of the new node to the root, and then delete the new node.
Or, you can do an admin dump and put in params to discard everything after the favored version, then reimport into a new repository and rename accordingly.
I've only done this in the lab but you can change the contents of /path/to/repo/db/current file to the desired rev and it seems to do what you ask. Only done it a couple times but new commits did work. Be good to make a backup first.