Subscriptions bought on the web
Go to Settings → Subscription and select Manage Billing. You'll go to Stripe's billing portal, where you can change plans, update your card, view invoices, or cancel.
If you cancel, your plan stays active until the end of the period you've paid for, then doesn't renew.
Plans purchased on the web come with a 14-day refund guarantee: cancel within 14 days for a full refund.
Subscriptions bought in the iPhone app
Apple bills these, so you manage or cancel them in your Apple ID's subscription settings: on your iPhone, open Settings → your name → Subscriptions. In Homvio's own Settings → Subscription, Manage in App Store takes you to the same place.
Apple handles refunds for App Store purchases.