Upgrade of 5.5 to 6.5U1 hosts

We have upgraded the vCenter in our test environment to 6.5U1g.  The ESXi hosts are still on 5.5.  Looking to upgrade the hosts to 6.5u1.  What is the preferred method to do that (Update Manager, command line). Eventually in our production hosts, we will need to take the hosts down to patch firmware.  Both of our test and work environments have the capability to do vmotion.  Just want to simulate the process.

Thanks,
TT