The advance has two screws holding it in place. Pull them and wiggle the linkage out from under the plate. You should be able to change it in place.
There are different advance units, each is marked with a number 8, 10, 16, 14, etc... The stamped number is the amount of degrees it will pull in under full vacuum. If you with a couple of numbers, it should work.
http://www.corvette-restoration.com/resources/technical_papers/Vac_Adv_Spec.pdf Here is more then you ever wanted to know about advance units.
