Steps:
- Go to Settings → Billing
- Click "Upgrade" next to the plan you want
- You're taken to Stripe Checkout
- Enter your payment details
- Click Subscribe
Your plan upgrades immediately. The difference in price is prorated if you're mid-billing-cycle.
Downgrading: Click "Change Plan" in Settings → Billing. Select a lower plan. The downgrade takes effect at your next billing date.