8.4.13 to 8.5 patch 1
You have to update to AM 8.5, primary first, then replica(s) one at a time. Make sure replication is synched.
Then you can apply patch 1 (or patch 2 is available) to AM 8.5, again primary first. Then replicas 1x /time. Patches can be rolled back, so you could take your AM 8.5 P1 system and roll it back to AM 8.5 base. But Service Packs and minor versions cannot be rolled back.