Repair of a torn rag top depends some on age and decomposition of the fabric. Some older fabric may be decomposed to the point where is has very little strength and may crack or crumble with flexing. There is not much you can do with that situation other than disposal and replacement with new fabric.

When the cloth backing is in decent condition, I have good luck with iron-on patch. Apply the patch on the inside. For canvas top that may be all you need. For vinyl top you can also apply textured "Liquid Leather" and/or vinyl repair the outside.

If the tear is in an edge location where you do not have easy access to the rear side of the fabric, then you may have to get the fabric loose from the frame. If you pull the front end of the fabric away from the wood bow (pull out tacks or staples), then you can apply iron-on patch to reinforce and extend the front end of the fabric when it's time to reinstall it.