Best to give the year on that SD.
For the early ones it may be as simple as adjusting the Bowden cable. On the 81-85, don't know about the later cars, the Bowden cable is mounted on top of the valve cover. It runs down to the trans. Has an adjustment at the point where the cable wire runs into the shielded cable itself. Just lengthen the cable right there to get the trans to shift later.
Otherwise there are a lot of complex adjustments to make on the vac system. You should run a search for those. Best article was written by Steve Brotherton. Explains all the details.