You should first access your Digital Dashboard. This can be done by going to "My Account", to the top right, then clicking on "My Digital Marketing".
Click on Upgrade in the top panel/ Compare Plans.
The package picker will appear, select the desired plan by clicking Choose Plan.
Choose a billing then click Continue to Payment.
This adds the new plan to the shopping cart, click checkout then fill in your billing details.
You have now upgraded your plan!