Just snapshot the appliance(s) before you upgrade. If you need to revert, revert to the snapshot.
Hi,
Basically a crash-consistent snapshot will do the job , but if you want to be 100% sure about consistency , login to the admin consile (https://vrops.fqdn.name/admin)
take offline the Cluster , create a vm snapshot and then upgrade your vrops appliance.
Good Luck!
A couple of things I would add as cautions when backing up vROps:
dcsvmware
See my blog post about how to do backup of vRops - The same is true for snapshot.
vRops Backup done right - Michael Ryom