Weapons can be upgraded in two ways, through blueprints and upgrade slots. Both blueprints and slots can be found hidden in various locations. Every weapon has upgrade slots. Higher quality weapons will have more upgrade slots. If a weapon has an empty upgrade slot, go to your inventory and select the upgrade mod you want to apply to it. Different upgrade mods increase different stats. The following is a list of all upgrade mods and their respective upgrades.

Assassin: Handling (Level 1)
Barbarian: Damage (Level 2), Durability (Level 2)
Berserker: Damage (Level 1), Handling (Level 1)
Brawler: Damage (Level 2), Handling (Level 2)
Bruiser: Damage (Level 2), Handling (Level 1)
Brute: Damage (Level 2), Durability (Level 1)
Champion: All Weapon Stats (Level 1)
Clicker: All Weapon Stats (Level 2)
Crusader: Durability (Level 1), Handling (Level 1)
Duelist: Damage (Level 1), Handling (Level 2)
Enforcer: Damage (Level 1)
Fencer: Handling (Level 2)
Gladiator: Damage (Level 2)
Juggernaut: Durability (Level 1)
King: All Weapon Stats (Level 2)
Knight: Durability (Level 2), Handling (Level 1)
Mercenary: Damage (Level 1), Durability (Level 2)
Paladin: Durability (Level 2), Handling (Level 2)
Pit Fighter: Damage (Level 1), Handling (Level 1)
Templar: Durability (Level 1), Handling (Level 2)
Titan: Durability (Level 2)

Blueprints do not require an upgrade slot on a weapon. However, they do require various crafting ingredients. Blueprints can add the following modifications to a weapon.

Burning: Damage over time to living or infected characters.
Toxicity: Damage over time to living or infected characters.
Bleeding: Increased damage output.
Electricity: Increased chances of stunning an enemy.
Impact: Increased damage output.