The button on the floor is the headlight dimmer switch. If there is a small brake pedal, there has to be a release handle right above it on the bottom of the dash, but they didn't use them until '69. The earlier models all had a T handle you pulled and twist to release..
"Also called emergency brakes or "e-brakes," they were designed to keep a Mustang secure when parked. The parking brake is also supposed to stop the car if the hydraulic brake system fails. Count on reduced stopping distances and loss of control if that's the only choice.
Right off the assembly line, '65-'68 Mustang parking brakes offered marginal safety. Dash-mounted hand brakes have never been effective, even under the best of circumstances. They aren't as useful as between-the-seat hand brakes either, due to mechanical advantage. To be valuable at all, a dash-mounted hand brake must be in the best possible operational condition. The '69-'73 foot parking brake is more effective, thanks to the leverage and the strength in the left leg.
Early Mustang parking brakes consist of a ratcheting handle and a shaft, a detent, a pulley, and a series of cables connected to the rear brakes."
The parking brake/emergency brake is self explanatory.. used mainly with standard transmission cars...
You can pick up a copy of the owners manual on ebay or amazon.com