By default the DAM pages have the functionality to mass edit the content. To get start, you'll need a selection of entities first, that you would like to mass edit. Create a selection by dragging and selection entities.
After the selection you will get the selection context menu appear in the upper right of the screen. There you'll see the number of selected entities (see the green box). Also you will get the option to mass edit the data, see the icon in the orange box. When clicking on the icon you will go to the mass edit page.
On this page you will be able to mass edit the selected entities. Make the changes that you want and don't forget to hit "Save" to store the changes.
This is of course a good option when the selection is limited. If you want to do a lot more entities at once, you can use the mass edit via Excel. In the following video you will see how to do this.