First, head to Subscription Settings and click the three dots next to the monthly seat you'd like to convert to annual billing. Click 'Switch to Annual Billing' and you'll receive 1 month free on your subscription!
Don't forget to upgrade your Integrations too!
First, head to Integrations and choose "Upgrade to Annual". You do not need a Promo Code to upgrade. Select "Upgrade" for the Integration you would like to switch to Annual billing.