You can upgrade your purchase either to a package with more activations or from a single add-on or the All Access offer paying only the difference.
Just log in to your account using the email for the purchase. Scroll down to the License Keys section.
Click on View Upgrades next to one license from the purchase you want to upgrade from. Now, select the license you want to upgrade to.
Upgrades are only possible for active licenses. If your license expired, you need to renew it first (for a discounted price) before the upgrade option is available.
An upgrade will inherit the expiration date of the purchase you upgraded from.