You've got two choices,
either have a new sheath made for it (a proper wet-moulded one).

or you can try to do a bit of wet moulding on your current one, assuming that it is made of veg-tanned leather.

If you want to give it a try, wet down the sheath and wait a bit for the water to fully soak through. Then wrap your blade in cling-film (saran wrap) and put it back in the sheath.

While the leather is pliable, work it around the blade and up over the bolster/guard area. Then let it sit until the moisture has evaporated. It should be nice and firm and properly fitted to your blade.

Good luck!

ith: