On early 944s and the 968 Sport and Club Sport there is little in the way of seat height adjustment so here is a way to acheive this. (This is an amateur solution and no gaurantees are associated with this suggestion.)
First of all, decide how much higher the seat needs to be.
You will need a 6mm allen Key,
If you slide the seat forward and look at the base of the seat you will see 2 runners attached to the chassis of the car.
The seat can be removed by undoing the cap head bolts with an Allen Key.
Slide the seat fully back and do the same at the front.
You should now be able to remove the seat.
To increase the height of the seat have 4 aluminium blocks made of the correct height and drilled a clearance size for the bolts. A local engineering company should be able to help.
You can also ask the engineering company for replacement cap head bolts of the correct length, This will be the length of the original bolts plus the additional thickness of the aluminium blocks. These are generally in increments of 5mm length and you must use high tensile cap head bolts.
It would be a good idea at this point to lubricate the seat runners.
Replace the seats by sliding the seat runners fully forward on the base of the seat and locking them in place.
Place the seat into the car and align the front holes for the bolts, you can place the blocks under the runners and loosely screw in the front bolts.
Slide the seat forward making sure it is even both sides and position the rear of the seat with the blocks inline for the holes and loosely screw in the bolts.
When you are happy that everything is lined up the bolts may be tightened fully.
I did this on a 1984 944 Lux (my first Porsche).
The 968 Club Sport has four bolts for adjusting the seat height at the side of the seats so try those first before taking the seats out.