How do i fix the speedometer on a 95 isuzu rodeo

No engine stated (size or cylinder counts)
nor if odometer works or not.

one engine has cable head other not.
so....
if odometer is dead on one engine, its VSS dead
if od0meter is dead on other , the cable is bad.
there.

the odometer is that counter deal on the cluster bottom tells how many miles car has moved.
it rolls, 1/10s as you drive, it the counter is dead
that means the odometer is dead. too.

Yours is cable driven for the most part. You may have a sensor at the transmission. Transmission sensor will 2 to 3 inches long. It will have a cable and electrical plug going to it. The cable will round off at the transmission or cluster. It should be square., Both ends. The sensor can be checked with a hertz meter. Also need to check for power and ground at the plug. Could be a bad wire.