In the top navigation of https://www.adobe.com/ there's the menu item "Buy". Select the type of business, then select your country and the product. There might also be numerous resellers in your country that will gladly sell you the upgrade.
Doesn't work. It's all greyed out. Been there. Try it: https://www.adobe.com/content/dotcom/en/products/catalog/software._sl_ id-contentfilter_sl_catalog_sl_software_sl_allsoftware.html?start=40