There is a spring clip on the tachometer that you have to depress that releases the tachometer cable housing. Look up under the dash with a flashlight and locate where the tach cable connects to the tachometer. You will see the spring clip.
Depress clip with a screwdriver and have helper pull on cable to release. When installing new cable, feed it through the firewall, find end of cable while looking under dash and line up with tachometer. Takes a little practice and patience but it can easily be done.
Hope this helps. Just replaced mine. Make sure to grease the tach cable with high temp on the bottom two thirds of the metal inner cable before installing.