How to reinstall the current version of macOS or downgrade it
Recovery reinstall, using an Installer app, or a bootable installer, or with an Apple silicon Mac in DFU mode? What the choices are, and how best to do it.
With OS X El Capitan, Yosemite, or Mavericks, you can use a USB flash drive or other removable media as a startup disk from which to install OS X. German guide → https://support.apple.com/de-de/HT201372