Weapons in Wild Hunt can be upgraded.
To do so, you have to enter the weapons screen and choose the weapon you want to upgrade.
The weapon is upgraded by levelling up its parts. After picking one of the parts, a window will appear on the left side containing all of the information on the given part upgrades, statistics and price of the upgrade.
The higher the level of an upgrade is, the more spare parts and coins/banknotes will be required to make an upgrade to the next level.
Upgrades will increase the overall weapon damage.