On the web
Go to Settings → Subscription and select Upgrade. Homeowners also get an upgrade option when they try to add a second home.
Choose monthly or annual billing, pick your plan, and check out with Stripe. Your plan is active as soon as the payment is confirmed, which usually takes a few seconds.
In the iPhone app (homeowners)
Go to Settings → Subscription → Upgrade to Multi-Property, or try adding a second home, then select Subscribe. Apple bills the subscription to your Apple ID each month.
Already subscribed on another device? Select Restore purchases in the same Settings section.
Service providers
Service provider accounts are free during early access, so there's nothing to buy.