Starting from the lower end of the serving to move the point down, or the top end to move the point up, put the edge of a piece of metal (NOT a sharp piece of metal like a knife edge) in between a couple of servings and push/pull to separate the servings (i find you can create about a one or two serving strands gap). Move up 3/4 strands and repeat, moving these into the gap you created. Repeat all the way to the other end of the serving.
As a tool, I use a pocket knife with a bottle opener. I use the edge of the bottle opener to press between the serving threads.
Note that after 3 or 4 of these full-serving moves, the serving may be quite loose, but you should have been able to identify the right nocking point, and so can now remove the serving and re-serve in the right place.
Note that I don't recommend this for the initial height setting when first tuning a bow - I simply use masking tape for that - this is fine tuning once you've got a rough idea where your nock should be.