An easy way to move back? No, not unless you have the installation media for a prior milestone.
You might get lucky finding a torrent for an older milestone, but otherwise you’ll have to wait for the next one. When new milestones are published the old ones are pulled from download.opensuse.org.
One alternative would be to use the automated Factory builds. That would give you a sort of Milestone 5.5.
I’m not sure what you or the project gains from you installing an old milestone and then deliberately not updating it though. You’ll need to be thorough and make sure you don’t report bugs that have already been fixed by updates you’re avoiding. Do you need the new kernel?