I have twice replaced the speedometer on my vehicle before and it is a do-it-yourself job. (I have a manual shift.) FIrst of all, purchase a Haynes or equivalent manual.
Unfortunately it is easy to rollback these speedometers. I purchased one from a junkyard (mistake) and rolled it forward to match the one removed from my vehicle. (which I kept and later had rebuilt as junkyard later exhibited same issue)
Go up under the hood, locate the speed-sensor and pull it from the vehicle with cotter-pin and speedometer cable still attached. Be sure to disconnect the wire harness from the sensor first. You'll have to remove one bolt and the retainer, then twist counter-clockwise as you pull to get it out. (this is due to gearing design)
Go inside of the vehicle. Remove all the bolts from the black plastic cluster... Three above the cluster and four on the bottom below the vents, left & right side.
Four bolts holding the steering column up to the dash will allow it to drop. I used a hammer to prop it up and prevent it from resting on the lower plastic paneling.
Now you can remove the black plastic paneling, but be sure to disconnect the power-mirrors and rear-defrost harnesses before totally yanking it out of there.
There are approximately four black bolts holding the cluster to the dash.
Gently pull the cluster from toward yourself. To help, you may want to go back under the hood and push the speedo-cable forcing it inside the firewall. Disconnect the wireharnesses from the top-left and top-right side of the instrument cluster.
Disconnecting the speedometer cable housing is tricky... So inspect the attachment closely and take your time to understand it... But you'll have to "rock" the black plastic retainer away from the brass-colored housing. This pulls the retaining pin on that retainer from inside the hole on the housing and allows you to then pull the cluster completely free of the housing, dash and vehicle.
If you're competent enough to have made it this far, disassembling the cluster is easy and needs no explanation. You'll simply remove a few screws, pull the black-plastic face away from the white frame, (leaving clear-plastic face attached) pull away all other gauges first and then the speedometer will practically fall out.
If you already have the original speedometer cable in there, leave it alone. You may want to pull it out, regrease it and reinsert it, making sure that the end that inserts into the speed sensor aligns perfectly. In any case, as you reassemble it, BE SURE that you do not damage either end of the square heads as you reassemble or replacement of the cable WILL be required; and that's just a pain because you'll have to measure and cut the new one to the correct length. I recommend that you make sure that the speedometer and cable are positioned to match before reattaching your cluster.