Upgrading Virtual SAN is a multistage process, in which you must perform the upgrade procedures in the order described here.
Before you attempt to upgrade, make sure you understand the complete upgrade process clearly to ensure a smooth and uninterrupted upgrade. If you are not familiar with the general vSphere upgrade procedure, you should first review the vSphere Upgrade documentation.
Failure to follow the sequence of upgrade tasks described here will lead to data loss and cluster failure.
The Virtual SAN cluster upgrade proceeds in the following sequence of tasks.
Upgrade the vCenter Server. See the vSphere Upgrade documentation.
Upgrade the ESXi hosts. See Upgrade the ESXi Hosts. For information about migrating and preparing your ESXi hosts for upgrade, see the vSphere Upgrade documentation.
Upgrade the Virtual SAN disk format. Upgrading the disk format is optional, but for best results, upgrade the objects to use the latest version. The on-disk format exposes your environment to the complete feature set of Virtual SAN. See Upgrade Virtual SAN Disk Format Using RVC.