2. OK, if you absolutely have to open the ring, go back to the last picot made (if there are no picots on the ring, go back one third of the way and between two double stitches) and slide the double stitches slightly apart so that you can grasp the shuttle thread , also known as the foundation thread or the carrying cord, with your shuttle's point, crochet hook or even a hemostat (clamp).
Pull gently on the shuttle thread until 1/2" - 1" protrudes. Then continue back on the ring to the previous picot and pull this slack back; then a third time at the starting point of the ring. Once this small of amount of shuttle thread is pulled through backwards, you can grasp it and pull it in the normal fashion to enlarge the ring.
3. Don't be tempted to try to wiggle the last stitch tatted loose and "untat" from there. This usually results in the last one or two half stitches slipping out of position and locking up completely. It also results in the thread breaking when you pull too strongly on it.
4. If you are very brave and experienced with the use of the hemostat clamp) you can actually go back to the start of the ring and pull the shuttle thread backwards through the ring. You must, however, hold your tongue just so and not breathe during this process. :)
Remember that every time you reopen a closed ring you are destroying the tensile strength of the thread. It is so easy to add on new threads and to tat over the tails or otherwise hide them that you really should not waste much time trying to reopen rings.