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Don't just patch over a torn seam with a topstitch - that's visible and won't hold the way the original seam did. If the tear is along an existing seam line, you can work from inside the garment by opening up just a small section of the seam on either side of the damage, then use a ladder stitch to close it back up invisibly. The key thing most people miss is making sure you're stitching through the actual folded edges of the fabric rather than just catching the surface, which is what gives you that bulletproof invisible repair.
A ladder stitch (also called an invisible stitch) lets you close a torn seam from the inside without unpicking anything - you slip the needle between the folded fabric layers and it closes the gap invisibly. If the seam's actually ripped open rather than just torn at an edge, you can also just restitch directly over the weak spot with a needle and thread, going back and forth a few times to reinforce it.
The biggest mistake is trying to reinforce a seam tear with a patch on top - it'll just pull apart again because you're not actually fixing the structural problem. If you're working on a seam that's split open, the ladder stitch approach mentioned already is solid, but what people sometimes miss is that you need to backstitch a few times at the start and end to lock it in place, otherwise it can unravel over time. Make sure your thread matches the original seam color and tension so the repair blends in.
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