You need to be an admin of Sage 50 also. 

Just double click on the file you downloaded to start the installation.. Make sure Sage 50 is closed when you do it.

Thank you for your response,
When I download the update, I do it with the Administrator user as well.

Thank you very much for your answer.
Well, in my downloads I don't see the file, it just tells me that closing Sage will install the update.