If you could do it without injuring the turtle, you could try to cut the plastic soda ring off the turtle, but the shell would be permanently deformed, if it looked like the RES pictured in the link. The turtle most likely got the plastic ring stuck around its shell as a young turtle and, as it grew, the plastic impeded the growth of the turtle's shell, not allowing the shell to expand normally, causing the cinching of the shell. The internal organs in the area of the cinched shell will be squished closer together than they would be in a normal turtle shell.
The best thing to do, if you found a turtle like this, would be to take it to a wildlife rehabilitation center, that accepts reptiles, in your area. A wildlife rehab center's goal would be to treat the animal medically and then release it back to the wild. If the animal can't be safely released to the wild, the wildlife center will often keep the animal, or find a center that can, and use it as an educational tool during educational seminars, educational outreach, and such.