Managing your subscription
Where you manage a plan depends on where you bought it, web, the App Store, or Google Play.
Billing shows your current plan and, depending on where you bought it, a different way to manage it.
Bought on web
If you subscribed through the website, Manage subscription opens Stripe's billing portal in a new tab, where you can change plans, update your payment method, or cancel. Prices are shown in USD, and you can switch between monthly and yearly billing from the Plans page before you buy.
Bought on the App Store or Google Play
A mobile purchase is managed in your phone's subscription settings, not on web, Billing tells you this directly and points you there instead of showing a portal button that wouldn't work.
On a team
If your access comes from your team, the subscription is paid and managed by your team owner and your calls draw from the team's pool of minutes. Seats are free, so there is nothing for you to buy. Billing shows this plainly rather than prompting you to pay for something you already have through your team.
Switching plans
You can move up or down between plans from the Plans page. Your new allowance applies from the next billing date, and minutes you have already used are not clawed back. A downgrade to a plan you already have access to (through mobile or your team, for example) is shown as already included rather than something to buy again.