First, you need enough blueprints to be able to initiate the upgrade. Then, you need to have all stats leveled up to level 4 (or 8 for the 3 star up). Finally, you need enough upgrade kits to pay for the upgrade. If you meet all three conditions, you can tap on the weapon card in the equipment menu, and then tap on the "star up" icon in the details pop-up.
How do I star up a weapon?
Last Updated: 38d