Users can upgrade their paid version by contacting firstname.lastname@example.org. You just need to pay the difference between the already existing license and the new version to be purchased if it is less than a month. However, if it is more than a month since your purchase, then there will be some extra cost to upgrade the license depending upon the time elapsed from the purchase date.
You can contact email@example.com to know your upgrade price and the upgrade invoice will be shared with you. Then you can proceed further to make the payment.