how to upgrade software on mac mini 2011 running high siera 10.13.6

High Sierra 10.13.6 is the highest version possible. See the following from Identify your Mac mini model - Apple Support

Mac mini (Mid 2011)

Model Identifier: Macmini5,1; Macmini5,2

Part Numbers: MC815xx/A; MC816xx/A, MC936xx/A

Newest compatible operating system: macOS High Sierra

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High Sierra is the end of the line for a 2011 Mac mini. It can not be upgraded to run a later macOS.

If you need to run a later macOS, then you need to consider buying a new or newer Mac.

You're welcome.