You must add the weapon you wish to use this with to your sheet separately.

At the end of each long rest you can select cold or the magical form of the normal damage type of this weapon, this weapon deals that damage type till the end of your next long rest.

Winter's Bite When you hit an opponent with an attack, you can sap their warmth. The creature must make a DC 13 CON save. On a failed save they take 2d4 additional Cold damage and have their speed reduced to 0 until the end of your next turn. On a success, they take half damage and their speed is not reduced. Once this feature is used it cannot be used again until you complete a short or long rest or you spend 5 minutes near a source of heat such as a campfire