How do I upgrade from macOS High Sierra 10.13.6?

how do i upgrade from mac os high sierra 10.13.6? i have tried following the info about it and iy keep saying my mac can not upgrade to 10.14 or further, what are my options. do not know much about mac technology.

can you help in any way?

my mac info: Model Name: Mac Pro

  Model Identifier: MacPro5,1

  Processor Name: 6-Core Intel Xeon

  Processor Speed: 2.66 GHz

  Number of Processors: 2

  Total Number of Cores: 12

  L2 Cache (per Core): 256 KB

  L3 Cache (per Processor): 12 MB

  Memory: 20 GB

  Boot ROM Version: MP51.0089.B00

  SMC Version (system): 1.39f11

  SMC Version (processor tray): 1.39f11

[Edited by Moderator]