If your Mac is capable of upgrading the OS, it will be an option in the Mac App store, installation is free. The App store may only offer you High Sierra or Mojave, if your Mac is capable. (It will offer you the highest version you can handle, and may not offer intermediate versions.) If you want to install a lower version than the App store is offering, but still an upgrade, there are instructions online to do that manually.
Mid-2010 should probably take at least High Sierra if not Mojave.
