If you want to downgrade your device, go to samfw.com and search up your device's model number. Than choose the latest update of the old software that you want to downgrade to, ensuring that the version you are in, has the same binary version. To check this, simply dial *#1234# and check CP. It will say U# (# is the binary version).
If it is the same you can downgrade. If it is newer, you can't. If you can, use a computer and get Odin.
Open Odin and extract the firmware ZIP files and put the files in BL, AP, CP, CSC and for older devices, it is a one file firmware and put it in AP. And that is it!