You can edit the ownership of Accounts, Business deals, Activities etc. from one user to another all at once using Mass edit of owner. The ownership change can only be done by an Administrator. How to proceed? It is quite simple :-)
Select the Mass edit of owner option from the left menu.
Now you just choose the original and new owner.
Tick the items which you want to change the ownership of.
Confirm all changes by clicking the Change owner button.