The track cover is just that, a cover. It should go on top of the existing track. If your track is too wide, then you might have to file down the sides of your track so the cover will fit.
However, I have never had to file any existing tracks down in all the years I've installed this type of cover, the 80-100
To secure the cover, us two of the 33-071
Pan Head Screws (#6 x 3/8). Install one on each end a few inches from the jamb. You don't want to interfere with the roller on the patio door. You will need a 7/64" drill bit and be sure to use an oil-based lubricant. The strainless steel track is harder to drill through.