As you already know, if you have ESXi5 free installed on host with more than 32GB of RAM, after upgrade to U1 you might get into trouble with license, see that post http://vmwaremine.com/2012/03/24/esxi-5-free-version-update-1-license-problems/
If you got that error, you can rollback Update1 from your ESXi host.
Access over remote console cards such as IMM, iLO, DRAC or direct attached screen:
- Reboot server
- Press Shift+r when the screen displays Loading VMware Hypervisor.
- New screen appear with ESXi build listed
- Confirm selection – Y
- Server restarts with an old build
Access to server via SSH
- Edit the files /bootbank/boot.cfg and /altbootbank/boot.cfg.
- In the bootbank copy of the file you should find updated=2. It will be updated=1 in the altbootbank copy.
- Swap the values for updated, save the files and reboot.
Note: the first way I mentioned using Shift+r is supported, the second method is not.