"SecureInfo_A" file for the desired region. Either dump it from another (N)3DS or find a "donor" file online (google). Make sure it is the right region or it will not work!
Gateway Launcher v3.2.2 (google).
DevMenu v6.2 (google).
WinHex (not sure if the free version works for our purposes, mine is licensed. Not like that will stop you google it).
I've been trying to change my 8.1J region for half a year now, I've tried every possible way. You're just out of luck as I am, there is no way to do it currently.