How to Unshrink: Cashmere (and other animal fibers)
The process of unshrinking cashmere (and other animal fibers) is fairly similar to the process of unshrinking cotton. First, fill your sink (or bowl) with lukewarm water. Then add 1-2 tablespoons (per one quart of water) of Borax or vinegar to the water. Add your article of clothing to the mixture and let it soak for 30 minutes, being sure to periodically stretch out the material while it's underwater. After it has been soaked, squeeze as much water as you can out of your garment. While we rolled our cotton garment in a towel, animal fiber-based clothing has to be stuffed with towels in order to return to its original shape. Again, stretch out the material as much as possible during this process. After, hang to dry.
If you haven't put that new cashmere sweater in the dryer yet, David Zyla, Emmy Award-winning stylist and author of Color Your Style and How to Win at Shopping, says there is another way to "rejuvenate" it. "Dampen the top half of the sweater using cold water in a spray bottle and place it flat on a towel. Then stretch the shoulders out wide and place a book covered in plastic or a towel to hold each one out as wide as possible. Leave it like this overnight in a cool spot and let it dry thoroughly," says Zyla. "[Then] try the sweater on. As most knits stretch out over time, you may find the fit now better than ever before. If the body still feels a bit snug, repeat the process of dampening and stretching each side with two books on each side, being mindful that each side is stretched the same amount. By the next day, your favorite sweater will be back in your wardrobe!"
How to Unshrink: Synthetic Fibers
Now onto synthetic fibers, like polyester and rayon. Again, the steps for how to unshrink clothes made of synthetic fibers are similar to the process of unshrinking cotton. Add 1 tablespoon of shampoo/conditioner (per one quart of water) in your sink/bowl with lukewarm water. Synthetic fibers only require 15-20 minutes of soaking. After, due to the sensitivity of the material, make sure you don't wring dry. While it is still wet, begin the (gentle) stretching process. Again, use pins or heavy objects to weigh down the garment while it is air drying. When fully dry, be sure to properly store your delicate clothing to preserve its quality. Taking good care of your clothes can go a long way, but—if you do have a mishap—use this guide to remove any stain.
How to Unshrink: Denim
Jeans are a whole different story. Denim doesn't usually shrink, but when it is blended with other materials, it's possible. There are two remedies to this one. The first one requires you to sit in a lukewarm bathtub for 15 minutes while wearing your jeans, then walk around/sit/periodically squat for an hour afterward (while still wearing the jeans) to reform its shape. If something about that sounds…uncomfortable, then you can simply use the same method as unshrinking cotton and hang to dry. Now that you know how to unshrink clothes, remember these 14 things should never end up in your dryer—or they might shrink, too.
How to Unshrink These 4 Types of Clothing, Source:https://www.rd.com/home/cleaning-organizing/how-to-un-shrink-clothes/