If you want to make changes to a group of tasks, you can use the multi-select feature in the Task Grid.

Just <SHIFT>-click on each task that you want to modify. You will see a yellow background for each of the selected tasks.

You can then drag and drop the selected tasks to a new grid number, or right-click on any of the selected tasks to duplicate, change visibility, delete, move to a new date, or assign to another user or group of users.