The towels are made of a microfiber, either 100% polyester or a blend of nylon and polyester.
If the odor is the result of the towel having been packed away damp, a day of hanging in the sunshine should help to eliminate the odor.
You can also wash the towel with normal detergent (avoid fabric softeners) or white vinegar and toss in the dryer (no dryer sheets) on low.
If the odor is the result of some organic material, it can be neutralized with a product called Revivex Odor Eliminator (previously marketed as MiraZyme).
If the odor persists after the above steps, please reach out to us here and let us know the size/model of the towel, when and where it was purchased, and your contact information.