Assuming the primary is currently active, the secondary will not go active unless you log into it and make it active - don't do this unless you have to roll back.
Take an export of the primary using the migrate_server command per the upgrade guide.
(optional) Test the import of this file in fresh R81.20 VM
Rebuild the primary to a fresh install of R81.20
Install recommended JHF onto primary
Import DB into primary
Make sure it all works (SIC is good, policies install, you get logs, all that), for however long you want. HA will fail so nothing you change will sync to secondary. The secondary won't be affected by this failing so you don't have to do anything about it.
Then if it's all good, fresh install the secondary, install same JHF and re-SIC it to primary.