Upgrading one host from HP-ESXi-5.5.0 to HP-ESXi-5.5.0-Update2

A vSphere infrastructure is made up by  3 HP Proliant hosts.

All the nodes were installed starting from the HP ESXi CDs.

Two hosts run HP-ESXi-5.5.0-Update2-5.xx.x, the last one is older and runs HP-ESXi-5-5-0-iso-5.71.3.

How can I allign the 3 hosts so that they run ESXi U2?

I cannot reboot for any reason the 2 nodes already running ESXi U2, while I can reboot if necessary the host running 5.5.0.

I presume I should use Update Manager, is it correct?

Is there a document or a KB article describing the steps to perform?

Regards

marius