You don't need to do it with 5.5 either, if you don't want to. As the others have pointed out, it's merely an alternative to the classic method of editing the settings.
And remember vCenter is just a server for centralized management. You can still perform maintenance by connecting directly to the ESXi hosts.
So, as Tim Scheppeit nicely explained, the easiest thing for you to do is:
- take a look on which host the vCenter is running
- shutdown the vCenter VM
- connect directly to the host with the vSphere Client and edit the RAM of the VM
- start the VM