DFU mode, which is a concept of Apple, is one of the concepts we need to know, even though it works a year. If we do not know this concept, which is generally required during software updates, we will have to take our device to Apple service.
If you are one of those who want to try the updates first, the DFU mod concept will be very useful for you. If we know this concept, we can go back to iOS versions that we don't like. We don't have to endure a burden of taking it to Apple service. As you can deduce from this short explanation, DFU mode is the software installation mode. When you switch to this mode, you can install your old version. Of course, before you need to have the version you want to install on your phone ready on your computer. If the software you are going to return is ready, you can go back to DFU mode and go back.