How do you repair a 2 or 3 ton automotive floor jack?

Put some hydraulic oil in it.

you need to put new seals in it undo the top of it then pull ti apart you will see how it is Dun

I'm assuming you want to repair it yourself? The most common problem i've run into is being low on hydralic fluid. If the jack is not leaking fluid and all the mechanical parts are in good working order, Take a screw driver and unscrew the fill plug on top of the cylinder, look inside, it should be full, if not add fluid to fill it to the top, reinstall plug, turn the knob at the the end of pumping handle clockwise until hand tight, the jack should rise unless you have a blown internal seal or a mechanical problem. good luck