Upgrade
Pick a higher tier in Settings -> Plan -> Manage subscription. Stripe charges the prorated difference and unlocks the new tier immediately.
Downgrade
Pick a lower tier. The current plan continues until the renewal date, then the new tier takes over.
Cancel entirely
Cancel from Settings -> Plan -> Manage subscription (Stripe customer portal). Paid features remain until the cycle ends. Subscriptions will not appear in iOS Settings, Subscriptions, because Apple is not the biller.