Within Teamwork, you can bulk edit tasks via the tasks breakdown panel in your project's Dashboard.
Go to the Dashboard area of your project and select the Summary subsection.
The tasks Breakdown panel is located to the right of the Summary view and shows the total number and proportion of active tasks that are assigned to each person on the project.
You also have the option to switch from a breakdown of active tasks to a breakdown based on estimated time.
To bulk edit tasks, click a user's name in the breakdown panel to open a quick view on the right detailing the user's corresponding tasks.
Click the Bulk Edit button in the top right to open a modal will open where you can select which tasks you wish to edit. Once you click Next, you can adjust which action to take (modify, complete, copy, etc.)
Depending on the action you choose, the modal will update with corresponding options.
When using the modify option, select the checkbox to the left of each property type before choosing corresponding values. For assignee and follower options, you can select one or multiple individuals, companies, or teams.
Click Update Tasks to apply your changes.
Fore more information, see: Bulk Editing Tasks