How do I downgrade my iPhone 5c from the newest update to the previous version?
If you mean downgrade from iOS 8 to iOS 7 for your iPhone 5C, this guide should help you.
Apple has stopped code signing iOS 7.1.2 for devices that support iOS 8. At best you can downgrade to iOS 8.0.0 which is likely more buggy than the current 8.0.2 version.
If you are having issues I suggest to perform a full backup and then restore the iPhone using the latest release.