Entering DFU mode is easy to be done, and you may need iTunes to check whether the iPhone has entered DFU mode. The tutorial is as follows: 1. Unlock your iPhone. Hold the power button and the home button for exact 10 seconds. 2. Release the power button, and keep holding the home button until the icon of iPhone shows up in the upper right corner of iTunes. You will see a circle at the bottom of the iPhone, and you have successfully entered DFU mode on iPhone now. But you won’t enter DFU mode on iPhone if you see the Apple logo after releasing the power button. You can start over and enter DFU mode on your iPhone. As is mentioned, under DFU mode, you are able to choose the version of iOS to update. If you want to do that, you’d better make an iPhone backup in iTunes, because you will lose almost all the data after changing the iOS on your iPhone. Therefore, you will need the iPhone backup to recover deleted files on iPhone.