How do I quickly upgrade my weapon in Warzone 2.0?

Create a kit with the weapon you want to upgrade
Use the double experience token for your weapon if you have one
Play the standard Battle Royale mode alone (without a Squad) or with friends
Find an area with a lot of safe cracking contracts
Crack the safe and buy the weapon you want to upgrade in the store
Spend the remaining money to buy armor plates – each plate you buy gives you experience for the weapon you are holding
Continue to complete the contracts with safecracking and spend all the money to buy armor in the stores
Repeat the break-in and purchase several times. If you run out of safes, complete other tasks