The green area in the pic is the vacuum modulator on the psgr side of the IP. The white arrow points to the IP. The red arrows show where there are two 3/8" bolts that hold it onto the IP. You can slightly loosen these (recommend a shorty wrench) and rotate the modulator clockwise or counter-clockwise to change the shift points. Be sure you only hand-tighten them back down. The modulator is plastic and will break, especially if it's cold out.
You'll have to ask someone else about using the vac gauge for adjustments as I've always just guessed by how it drives. Currently my M1009 shifts from 1st to 2nd at around 15-25 depending on RPMs and from 2nd to 3rd at around 35-45, likewise.
Good luck and Semper Fi.