How to return back firmware ; end how to change csc .. stock non root. please help.

How to return from firmware United Kingdom BTU G928FXXU2AOJ5 last firmware. to France XEF G928FXXU1AOH3 firmware. i tried meny times but failed.. end any time to flash btu firmware the csc it is not change. end after clear cache end clear cache factory reset. why;;

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