I have the basic plan and will be adding a second camera

I currently have a video doorbell and I am subscribed to the basic plan for $40 a year. I will be adding a stick up cam in the next few days. I was already charged last month $40 for my basic plan. Since I am adding a second device, I will need to upgrade to the plus plan. What are the steps that I will need to do in order to get credit for my basic plan which was charged last month and upgrade to the plus plan for $100/year to cover all my devices. I do not want to be subscribed to two plans. Also, when do I upgrade the plan after the trial ends or immediately once I install the new additional camera?