Why downgrade? There is nothing technically wrong with 9.15.
The official downgrade is documented. May need to pull a KB or find the upgrade/revert guid.
Basically, you have to step backwards through each major version
9.15 -> 9.14 - 9.13.
You may be better off just doing a CLEAN HALT of your nodes and find one of the many places I have indicated how to init a system.
I would just keep the 9.15