Chapter 2 of Fortnite is full of features. Among them, the upgrade of weapons via a workbench. Several are available at various fixed locations on the map and allow you — by using a specific quantity of materials — to upgrade the quality of your weapons by one level.
Map
You can find weapon upgrade bench at several locations. Other ones exist and we are in the process of finding them.
The amount of material needed depends on the rarity of your weapon.
The use of the workshop is very simple if you have the necessary materials. Just approach it and press E. After a short cooldown, a burst of green light will be visible and your improvement will be complete.