DFU stands for Device Firmware Update and is a mode that is normally used to allow iOS to be loaded onto an unresponsive iPhone. Here we're going to use it to force a clean installation of iOS.
To do this you will need access to a Mac or PC running iTunes. You can't do this from the iPhone directly.
To put your iPhone in DFU mode, you'll need to perform some button presses that feel like sleight-of-hand tricks, and these will leave you feeling like you need an extra hand or two.
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Follow the instructions carefully and take your time. It can be fiddly, and it might take you a few attempts to get your iPhone into DFU mode and recognized by the Mac or PC.
Once the iPhone is in DFU mode and connected to the Mac or PC, choose Update from the popup and follow the prompts.
If, while downloading the update, the iPhone exits the recovery mode screen, don't panic! This can happen after 15 minutes of inactivity. Allow the download complete on the computer and then put the iPhone back into DFU mode and reconnect it.