Q: how can I reset my iphone 5c to factory settings

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Settings->General->Reset->Erase All Content and Settings

Not sure what you are after. If you want to reinstall the iOS so you have a fresh operating system then do a system restore and select new, not from backup.

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support