Upgrading a resonator is subject to the same rules as deploying one, and is done from the same screen. There are two additional rules: you can’t upgrade any resonators until the portal is fully powered (all the resonator slots are occupied), and you can’t replace a resonator with one of the same or a lower level. Upgrading a resonator costs 50 XM per resonator level, just like deploying a new resonator. Upgrading your own resonators grants no AP, but you get 65 AP for upgrading another agent’s resonator. Every resonator you upgrade also contributes to your progress for the Builder medal.