License check date must be older than Service check date before upgrading. There is a good chance of configuration failure without this condition.
From CLI, run command “tmsh show sys license” (without quotes) to see your license and service check date.
To re-activate a license, go to device GUI you want to upgrade(standby device) and system --> license and re-activate button at the bottom of the page. This will let you re-activate license and hence license check date will be older than service check date. (tmsh show sys license).
You can continue with your next steps to upgrade..(hope you have a implementation plan). After upgrading standby device and making it active(failover) , do the same thing on other device.
Hope you already checked these articles..
https://support.f5.com/csp/article/K9476
https://support.f5.com/csp/article/K13845
https://support.f5.com/csp/article/K7727
https://support.f5.com/csp/article/K5903
Hope this helps!