This is something you need to have TAC assist with, as they have methods to generate temporary root access to the Linux OS and these files can't be accessed via the normal platform administration CLI commands.
You may also realize there are plenty of documented methods to gain root access on a Cisco CUCM server yourself, but my favorite, in that it includes good screen shots and Vi text editing keystrokes help can be found here: http://htluo.blogspot.com/2011/08/root-access-on-cucm-and-other-uc.html
Of course, you shouldn't do this. It's unsupported by Cisco, Cisco TAC, likely voids warranties and service contracts, could be illegal, and who knows what else. Don't do it.
When Cisco TAC is assisting you, you might find some of the documentation associated with removing the licexpiry.txt file is incorrect.
To find the actual location of the file,
find / -iname licexpiry.txt
This should return the real location of the file, something like:
With that information, to remove the file you can run:
Good luck, maniacs.
Edit 9/30/2013: see http://webmaxtor.blogspot.com/2013/09/upgrades-are-prohibited-during-license.html for some additional information on gaining access to the CUCM file system.