Please review this article for mass managing and exporting lists:
Currently, the Work Order grid allows for Mass Management for a number of Work Order Fields.
Assignee has now been added to this pool of options.
When inside the Work Order list you have created, click on the hand symbol to access the mass manage option.
Choose the assignments to mass update.
Choose the Assignee to update this field on the work orders.
Once the assignee name has been updated, the final steps would be completed :
Make sure the Mass Update screen reads that all of your records will be impacted. Press the Update button when ready.
Enter the email address the mass manage notification will be sent to and Select Proceed.
Once the update has completed, the email address will receive a notification stating the mass update has finished.