On a Mac, open Disk Utility.
On some older Macs, restarting and holding Command + Option + R may allow you to reinstall the version of OS X that originally came with your Mac through macOS Recovery . Alternative Downloads: ISO and DMG Images mac os x mavericks 10.9 iso and dmg image download
Open and run:
Insert your USB drive, select it in Disk Utility, and erase it with a compatible format (GUID Partition Map and Mac OS Extended). On a Mac, open Disk Utility
Finding an official, full installer today can be tricky because Apple no longer lists it directly in the main App Store. Official Update Files: On a Mac