A set of points has two main parts - the fixed contact, and the moving contact. The fixed contact is part of the frame of the points, which is mounted to the stator plate with a screw. The moving contact has a spring, and a block that rubs on the points cam.
If you look very carefully, there should be a little notch in one side of the fixed contact frame. The notch is adjacent to two little pegs, which are part of the stator plate. The notch and the pegs work together as a "pry-point" that lets you open/close the point gap. Slack off the screw a bit (don't take it fully loose, just enough that you can move the fixed contact) and use a screwdriver in the pry point to increase or decrease the gap. Then, retighten the screw.
More gap = advancing the timing, less gap = retarding the timing. A tiny little adjustment can change the timing drastically, so trial and error is the rule. After you've done it a few times, you'll get a feel for how much adjustment is required to change the timing a given amount. It's very sensitive!