For a quick glance here is a video - R/W and R Check Boxes at the top of the Permission Access Page
You can now select all the R/W(Read/Write) and R(Read Only) checkboxes at ones for all the permissions under the Admin groups.

When you check R/W, it will select all the permissions appearing per page.

To enable all the permissions you will have to change the items per page to 1000.

Don't forget to save the changes!