Bulk Edit Task Items on any Task List

"Contacts", "Approvers", "Predecessors", "Followers" are not fields that can be bulk edited.

Select any task list to open it.
Select multiple tasks using the checkbox to the left of task title.
Select "Bulk Edit" button (this will appear when tasks have been selected), then select "Edit"
Use the field dropdown to make a field selection.
Next, enter the value that should be applied to all selected tasks, then select "Save".
The task table will be updated dynamically with the new values.

Make batch changes to tasks