Where
Settings → Billing. The plan card holds every plan control.
Upgrading
Choose Compare plans, pick the plan you want, and complete checkout. An upgrade takes effect immediately, and you are charged the difference for the rest of your current period.
Moving to a smaller plan
Also from Compare plans — choose the smaller plan. A move down is scheduled for the end of your current period, so you keep what you paid for. Make sure you are within the new plan's included users first.
Monthly or yearly
When switching is possible for your plan, Switch to yearly billing (or Switch to monthly billing) appears on the plan card and shows what changes before you confirm. Yearly billing lowers your effective monthly price. See the pricing page for current plans and prices.
Payment method and invoices
Those live in the billing portal behind Manage subscription.