It is mounted to the instrument cluster from behind. Need to pull the entire instrument cluster in order to get the bezel off.
To pull the cluster place a piece of tape over your column and the dash underneath it (so you don’t get scratches). Then if the car is a column shift engage the e brake and put the car in first. Then pop off the indicator and it’s housing. If floor shift skip that step. Then remove the two screws from the top of the bezel that go into the dash. Lean the cluster forward slightly and reach behind the cluster to remove the speedometer cable. If you have tiny hands this is easy. If not reach under the dash and go that route or if all else fails detach it from the trans and give the line some slack. Once the cable is off then release the harness clip and seatbelt light. Don’t force the cluster, they are fragile... once it’s out remove the four screws from the back and the front bezel comes off.
when you lean the cluster forward there are two clips at the bottom, raise the cluster straight up to disengage them. there is a speedo cable clip on the firewall under the hood, below the steering column (if not previously removed) that will give the cable some slack. when you replace the two bezel screws...don't over tighten them. the clear lens are Mustang repops...
On these old cars, especially here in the rust belt, I would not suggest putting the parking brake on unless you know it works and will disengage.