You will find different approaches more detailed in the guide. One option could be:
0) Backup your environment and dont make policy changes during the upgrade process of the cluster
1) Upgrade the first gateway and install the latest JHF
2) Change Cluster Object in SMS to R81.10 (you may also need to upgrade your SMS and Smart Event first)
3) Activate MVC
4) Install the policy from your SMS on your cluster and select the Install Mode "Install on each selected gateway independently". The installation will fail on the R80.20 gateway, but should be successful on your R81.10 gateway.
5) Now you can use clusterXL_admin down on your R80.20 node, because MVC in enabled. If you try to use it without MVC the command will not work (at least on R80.40 it did not work)