Samsung made this wonderful guide. I'm just augmenting it to fit the new issues we've encountered editing it.
You'll need:
Absolut Operator Replacer 3.1
SEEM Editing Knowledge (in some cases)
P2KMan (in some cases)
XVI32 (in some cases)
OpEdit (in some cases)
First, connect your phone to the PC, open AOR 3.1 and
click on "Connect". Then, Write the "thing" you want to put instead of your carrier name, and
click "Patch" After patching is done,
click on "Restart" and close the program.
Then, check if the program changed your carrier name.
IF IT DID CHANGED IT, DONT CONTINUE READING. YOU'RE DONE. 
Sometimes (i've noticed that this happens with newer models) it wont change. This is due to the fact that the program does 2 SEEM edits instead of the necessary 3 edits. Sometimes, it wont even make the 2 edits. So, time to open the SEEM and check it out.
Open P2K Manager and download SEEM 0032_0001 and open it in XVI32.
Check for the next offsets:
Offset 4B, bit 6 must be set to "0"
Offset 02, bit 1 must be set to "0"
Offset 40, bit 4 must be set to "0"
Note that "0" means "unchecked"
An easy way to find the offsets in to press CTRL + G and then write the offset
It's recommended that you check your plmn_text_table file as well. Download it, open it in OpEdit and make sure all the operator names were changed. if not, do it manually. (normally, it doesnt edit a couple of them) Upload the file if you made corrections.
After making sure you made the necessary adjustments, everythings OK.
Save the SEEM, upload the SEEM with P2KMan, and restart the phone. There, you made it. You have your Alpha Tag edited. 
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