To do the action over multiple workitems,
1. Select multiple workitems. There are two ways to select multiple workitems,
a. Press Shift & select multiple workitems at a time.
b. Press Ctrl & select workitem one by one.
2. Following selection of workitems, action menu appears on the top. Few quick input options are directly available. You can also access more input options by clicking on More Actions.
3. The popup appears on click of More Options. Do require changes and it will be applied to all selected workitems.