Don't do it.
According to Rory Cellan-Jones on the BBC tech site, upgrading to iOS 6.1 is causing alarming battery drain because devices keep on contacting the network.
Vodafone, worried that it could overload the networks, has warned Apple 4S customers not to upgrade.
Cellan-Jones said that Apple brushed off his queries, which is par for the course in Apple's current disdain for customers mode.
I did this myself some years ago on my iPhone 3. After upgrading the OS, the phone slowed down horribly.
Apple wouldn't tell me how to go back to the original OS but they put me in touch with someone who could and, after a complex process, the original OS was restored. All was well.
Apple is taking a terrible risk in treating customers this way.