Canvas shop
I had mine done at a canvas shop (not a sail loft; he mainly does biminis and car upholstery.) I had him do all the sewn-in slugs at once. The stitching has to be cut, and then the webbing resewn. I suppose you could DIY if you have a heavy duty sewing machine. It could also be done by hand, but it would take a long time. I was only charged $35 for all four, with me providing the new slugs, of course. He said he used the same type of UV treated thread as original, which appeared to be some kind of monofilament. It's lasted a year so far.