update does not work

A couple of weeks ago I received an iPhone 6 which had been reset before takeover. After I initiated the phone it upgraded to iOS 12.4.8 but after that there seem to be no further upgrade available. I get the message: No further upgrade is nesseccary (or something equivalent).

Is there anyone who can help me with hints on how to progress upgrade so that I can receive the latest (iOS 13.6), please?