If the light cannot be repaired without removing canvas, or the portin around the light, make a mental or photographic note of the lacing pattern used in your top and remove the canvas. When replacing repeat the lacing pattern. Should be able to do that pretty easily, I would think. Have never relaced one, but have gone over the lacing to pull slack out that developed from stretching. Not much, but hope this helps.
Find the center of the top string and make a small knot to hold center, than lay your canvas out on top of your top. Once centered, lock the canvas in place with a few zip ties.
Now find the most center Eyelit on the top in either the bow or Stern and run your center knot of the string from the top down and tie a large knot in it to hold it there permantly.
From there take either your port string or starboard (doesn't matter) and run it to the next eyelit going top to bottom, than run it over top the frame of the top, under and back through the same eyelit and run it to the next eyelit....repeat and do it evenly on both sides til you get to the end at either the stern or bow, which ever you decided.
I put new canvas on my t-top last year. I used tie wraps to center the canvas and then went around and adjusted them until the canvas was sitting centered. I then pulled the tie wraps tight and took the rope and laced it up.