A Fortnite Weapons upgrade system was introduced in Fortnite letting players upgrade their guns during a game game. But there is a cost to attempt to upgrade your weapons and it's more than just resources, it's time. Players need to spend significant looting time doing so which is why this exploit is so powerful.
And according to Redditor kewalts, it's rather easy to do.
Fortnite Exploit Allows You to Quickly Upgrade Weapons
To make this exploit work, you'll want to start with your lower-tier weapon so an uncommon one before a rare. As your character is going through the upgrade progress bar, at the last second, swap to your higher-tier weapon, and if you time it correctly, it will upgrade the weapon instantly.
This isn't a ground-breaking exploit, but it's clearly not as intended and Epic Games will crack down on it eventually. Until then, practice your timing and you'll be upgrading your weapons in no time.