You can view and upgrade your OpenAssistant plan from the Billing section of your account:
You can also get there by going to Settings → Admin → Billing.
Upgrade your plan
Go to Billing in your Admin settings.
Choose the plan you want and select either monthly or annual billing.
Click Upgrade.
If this is your first time upgrading, you'll be prompted to enter your credit card information.
Complete your upgrade.
Your account will be upgraded immediately, and the credits included with your new plan will be added to your account. You will be charged at the time of upgrade based on the monthly or annual billing option you selected.
Using a promo code
If you have a promotional code, click Have a promo code? below the available plans before upgrading.
Enter your code, and the applicable discount will be reflected in your subscription.