Basically, you need a flash tool like Odin Flash Tool and the correct/exact firmware for your device. Both of which you can easily download from the internet. You will need a computer and a USB cable (best to use the original cable that came with the device) to connect your device to the computer.
As for the procedure on how to do the flashing, again, you can do a quick search on that in the internet. There's also YT in case you need a visual tutorial on how to go about the flashing process.
Good luck.
PH