Where
Settings → Billing → Manage subscription. Choose the plan you want.
Upgrading
Takes effect immediately, and you are charged the difference for the rest of your current period.
Downgrading
Takes effect at 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
Yearly billing lowers your effective monthly price. See the pricing page for current plans and prices.