When you want to undo some stitches. First, pinch the stitches you want to keep with your right hand to prevent them from being undone accidentally. I want to keep this one stitch, so I pinch it.
Then pull the working yarn slowly with our left hand until the hole for the hook is pulled down. Now we can see that there are two loops, continue to pull the working yarn slowly until only one loop remains. Now we have undone a stitch.
If you want to re-crochet this round, you can undo stitches to the very first stitch. It's the stitch where the marker is. Use your fingers to pinch the 1st stitch we want to keep. Then pull the yarn with your left hand until there is only one loop left for the hook. You can then put your hook through this hole and re-crochet.