Review the upgrade guide that weinstein5 posted.
Absolutely have a good backup first.
Check Hardware Compatibility
If the manufacture has a custom ESXi image for those servers I'd use that.
Shutdown all VMs on a host and perform an upgrade install(you could also use this as an opportunity to update firmware)
Check that everything comes back up as normal
Upgrade VM HW
Optional: vCenter on new VM
Optional: VSA implementation if you have licensing for HA or vMotion