How to configure retention on Enterprise Vault

Hi,

I have a simple question, I need to creat a policy, or something like that, to just retain 1 year from a specific mailbox (the defaul of my system is 3 years), I would like to know how is the best way to do it.

I´m using EV 10 and my I´m using exchange 2007 Ent sp2

Thanks