Firstly ensure that the "Enable DAT file downgrades..." option is enabled in the Updates section of the agent policy, and that all the required client machines have received this policy.
Then, assuming that the clients are currently configured ( in the same part of the agent policy) to update their dats from the current branch, when you want them to roll back, you have two choices:
1) Check an older DAT into the current branch (or move the DAT from Previsou to Current) and run an update task: or
2) Change the policy so that the clients get their DATs from the Previous branch, wake up the clients, and run an update task.
Generally speaking option 1 is easier.