I have a paid EMclient license (windows 8.1) and I can’t update to version 9.
My wife has the free version (windows 10) and she was enable to upgrade…what the heck ???
That is because you purchased a regular Pro license, so for version 8 only.
If you want to upgrade to version 9, you will need to purchase a version or lifetime upgrade option.
You can do that here. There are options at the bottom of the page.
What ??? A free !!! version can update and a 50 euro’s paid one can’t ???
How is that possible ???
My high appreciation for EMClient suddenly exploded ! 
A Free license can’t be used for business, it can only have two email accounts, many of the Pro features are disabled . . . . You can find most of the limitations listed here.
If you are using eM Client for personal home use only, and don’t need the Pro features, you are welcome to register and activate a Free license.