In Operate you can use the mass update option to assign or re-assign records to specific users.
To do this, navigate to the module you wish to assign the records from - e.g to reassign tasks go to tasks for Opportunities go to Sales>Opportunities
- Select the records you wish to update (1)
- Click on update (2)
- Select owner from the list of available fields (3)
You can then choose a new owner from the list of options in the dropdown (1)