Easiest way to update jails from 9.0 to 9.1

[other at ahhyes.net]

Hi Guys,

I have a machine running 9.0-RELEASE with about half a dozen jails. I 
notice FreeBSD 9.1 has now been released, so I am wondering what the 
easiest way (less headaches) is to update all of my Jails.

I'm familiar with running buildworld and makeworld to upgrade an entire 
machine, I am just not sure what the deal is with the Jails.

Any hints appreciated!

BR,
Alex.