Click the Upgrade button under the course on your edX dashboard.
You can also find the Upgrade button on the right side of the course homepage inside of the course to access the checkout page.
Note: If these buttons aren't available, then the upgrade deadline of your course has already passed or this course does not offer certificates.
On the payment page enter your credit card information or click the PayPal button in the upper right to pay from your PayPal account. All fees are charged in US Dollars.