Navigate to the official Apple "How to download macOS" page.
Once the "Install macOS Sierra" app is in your Applications folder, you can create a bootable USB drive for a clean installation. A USB flash drive with at least 16GB of space. The drive will be erased, so back up any existing data. The Process: Connect your USB drive to the Mac. download macos sierra 1012 dmg file install
Open and format the drive as Mac OS Extended (Journaled) with a GUID Partition Map . Name the drive "SierraInstaller". Navigate to the official Apple "How to download macOS" page