You need to go onto Ruckus site, download the 200.8 for your AP models, R510s Unleashed. Keep it on the same local computer. Then on the local computer, log into the unleash AP, probably default password, then do upgrade from the service menu. It will upgrade one AP at a time...Thats kind of an overview.
When i was testing 200.9, the configuration backup data from 200.8 was not compatible. So, now that you going down rev, you may lose all your configuration. probably call up Ruckus and demand them to down grade you, as 200.9 have major reliability issues.