describe the dependancies.... ESXi Version = vCenter Version as well (whole vSphere).
As well you can find the release notes with current supported plattforms here:https://www.veeam.com/veeam_backup_9_5_ ... tes_rn.pdf
As well you find the Veeam Update Pathes there.
Keep in mind that you need a new license file for each major version 8.x or 9.x for Backup & Replication. If you are under maintenance you can create it on the Veeam Website in your licensing portal (just login at Veeam.com).
So you need to update the B&R Server first.