How to get iOS device from self-enrollment to DEP

Our Apple DEP certificate had expired and we had 56 Apple iPhones self-enroll in our MDM server before we realized it and now they are not Supervised. I have renewed the Apple DEP certificate and want to know the process to get those 56 iPhones enrolled through DEP so that they can be Supervised. I found the following statement in a troubleshooting section, "if the device has been enrolled in the MDM server using an enrollment method other than DEP, remove the device from management, reset the device and sync again with the server. The device will get listed on the DEP page." What are the exact steps I should use to remove the device from management and then what's the best way to
reset the device? Thanks!