Is it possible to mass update tasks?

Yes, you can perform updates on several tasks at once. Simply highlight a few tasks by clicking them with the CTRL key pressed, then right-click any of them to use Edit # tasks:

To make mass edits, please use the import functionality. For as long as you provide IDs of tasks already present on the board, the imported information will overwrite current data placed in them. It’s a very good way to manage changes across a large number of tasks in one go.