Updating Jails?

[Konrad Neuwirth]

Hello everyone, 

the handbook on the website is very helpful in regards to jails, and I was able to set up a jail or three quite quickly following the instructions there. What I do not yet understand, though, is what the process would be to update a jail once you decide to update the hosting system. My presumption is that you are not well-advised to be running a jails userland that differs from the parent; but what would be the most secure (and convenient) way to update? (Maybe this question also might be worthy of addition to the handbook). 

Thank you very much, 
 Konrad