Evenyone who recently started working on Web PD will come across this there is no revisions lower left section in Web PD using which you can click on a previous snapshot and open it. There is an alternate way to acihieve that in Web PD, you will go to Web Process Center and click on an application and a list of all snapshots will appear, but next to each snapshot you will see a open in designer link which will open that specific snapshot in Designer, then you cal right click on an artifact and copy to tip or revert in tip.