Is Plesk responsible for ASP.Net update?
How to update ASP.Net version via Plesk?
No, Plesk is not responsible for the ASP.Net updates. ASP.Net updates are obtained with Windows updates.
Optionally, the update can be performed manually. The instructions can be found on ASP.Net download page, for example: Migration Guide to the .NET Framework 4.7, 4.6, and 4.5 and .NET Framework Versions and Dependencies