We use a heat gun on low setting, but a hair dryer on high setting will also work.
If the sheath is too loose - Take the knife out of the sheath, aim the heat gun or hair dryer at the portion of the sheath that should catch your knife and keep it from falling out. Don't get the Kydex too hot. If you do, you may cause the sheath to lose its shape entirely. Keep the gun/hair dryer about 2-4 inches away from the sheath. Aim for short bursts of 5-10 second increments. The Kydex does not need to be completely soft, just soft enough to be barely pliable. Push on the part of the sheath that should catch and hold the knife from falling out. Only small adjustments are needed to tighten the sheath. Once you feel it's tight enough, put the knife in to check the fit (again, make sure the kydex isn't too soft). If it's tight enough, remove the knife from the sheath and let the sheath cool. If you leave the knife in while it's cooling, the Kydex will likely loosen again while it cools.
If the sheath is too tight - Follow the same steps for sheaths that are too loose. However, instead of pushing in on the Kydex, simply insert the knife into the sheath and remove it. Repeat this 2-3 times until the knife comes out more easily, but still locks into place.
