Yoga and lots of any stretching for the range of motion you're trying to increase....and lots of high quality Collagen protein and
quality whey/vegan protein...if you want to increase your flexibility quickly do lots of yoga and ingest a high amount of supplements that keep your ligaments and connective tissues and muscles as healthy and rapidly healing as possible
(collagen, hyaluronic acid, and other QUALITY proteins).
When you stretch you induce micro-tears in your muscle fibers etc. and you want those to heal as quickly as possible and create new range of motion but its even more important to keep your joints [pun "not" intended

] and connective tissues healthy with the supps. already mentioned...and your body should recover and get more limber, more quickly...theoretically...if you actually stretch and take those supplements....and stay well hydrated.