Changing your plan
Upgrade or move to a smaller plan in Account → Billing and plan. You see what it costs first.
In Account → Billing and plan you choose another plan for a product. Before anything changes you see what it costs. Once you confirm, the plan changes right away, and payment is prorated for the time left in the period.
Each product (receptionist or Office Manager, Lead Caller, website assistant) has its own subscription, and you change each one on its own.
Your invoices are listed in Account → Billing and plan. Downloading them and updating payment details happen in Manage subscription, which opens in Paddle.