Make that backup now. Then read the whole tutorial before you start so you know what to expect. Make sure your phone has plenty of battery before starting too!
There are a couple of ways to do this. I find the easiest way is to use a Samsung factory image. If you cannot find one on Samsung’s website, try the XDA Developers Forum here.
Navigate to the Samsung firmware website.
Enter your device model number into the box and find the correct firmware version for your device. Download it into your computer.
Download Smart Switch from Samsung too and install it.
Download Odin and install onto your computer.
Connect your phone to your computer so it is recognized and set to file transfer.