After going through many useless posts to edit externals for a repo, google finally helped me to reach atsome useful point. I thought this may help many.
propedit to change externals on a repo.
~Checkout the repo on your machine.
>>>svn co svn+ssh://.....
~go to the directory which contains sub directories coming through external links.
>>>cd dir_containing_external_dirs/
~execute the command that will edit external links
>>>svn propedit svn:externals .
don't miss dot at the end.
~checkin your code.
>>>svn ci -m ""
~update your code with new external links.
>>>svn up
