You should replace your focusing screen, it cannot be repaired. Replacement focusing screens can be found from a variety of sources, ebay lists several, at around $20 for the original Canon part.
There are several different replacement screens available. The original standard screen fitted to the 70D is the Ef-A. A popular non standard screen is the Ef-D which has grid lines added. If you like to use vintage lenses without autofocus, you might prefer the Ef-S screen.
Replacing the screen involves popping out a retainer clip while holding the camera upside down, you can then lift out the screen with a pair of tweezers and put the new one in place. The replacement screen comes with instructions and a pair of plastic tweezers for handling. Wear latex gloves and avoid touching the surface of the screen, it must be handled only with the tweezers supplied.