possibly an 1/8th inch or smaller in diameter flat tipped punch, but you'll have to confirm what will fit (it is too hot to walk outside and check this at my car), put the tip in the hole in the center of the rivet and strike the punch with a hammer but do not use too much force. If you do it right the pin somehow releases the tether.
You use the hammer then striking the top of the rivet to lock/secure the new tether.