Some times it is required that you need to boot a router from scratch. It is difficult to find appropriate version of the IOS software that is compatible with your router hardware. Therefore, it is recommended that you take a backup of the IOS boot file and restore the same when needed. A step by step instructional guide is given below:
https://www.computernetworkingnotes.com/ccna-study-guide/cisco-ios-backup-and-restore-process-step-by-step-guide.html