I tried but could not find <MediaCreationTool21H2.exe> and concluded that '21' in the file name referred to the year, i.e. 2021. So, I downloaded the <MediaCreationTool22H2.exe> and ran it as admin. Everything went well until the phase where the OS was being installed. At some point, the blue screen appeared with a very meaningful message: "Your device ran into a problem and needs to restart..." (I was too slow and did not photographed the screen.) The "device" did restart and went back to Win 8.1.
So, I made the decision to leave things as they are. Unless I discover that the unfinished upgrade created new issues, this tablet will live with the current OS. I should not take more of your time.
Thank you very much for you help!