Pull the vacuum line off of the modulator. When you look inside you should see a small slot that a screw driver will fit into. Use your flat screw driver and turn it clockwise to raise the shifting points, and counterclockwise for earlier shift points. From what you describe, you want clockwise turns. Don't turn it all the way in! Start with 1/4-1/2 circle turns, take it for a drive and see how you like it, and just fine tune it that way. Hope this helps.
Some modulators don't have the adjustment screw. If yours doesn't have it pull the modulator and make or buy a longer pin for behind the modulator. Longer pin = later shifts shorter pin = earlier shifts.