You can apply a promo code from Twilio to your existing account in two ways, either by upgrading your trial account with a specific button or by adding funds to an existing non-trial account. To upgrade, click the "Upgrade Project" button and enter the promo code in the required field after disabling automatic recharge. For existing accounts, go to the Billing section, add funds, switch to the "Promo Code" section, and enter the promo code to apply it. If you encounter any issues, contact Twilio Support for assistance.