You have to use a special editor and a hex editor.
Open a hex editor, search for the word "FONT" (Should show up 3 times). Move to the left by four bytes (So it displays "....FONT" in the text). Dump the binary data using said option from your hex editor's context menu. (You should probably increase the second non-0 hex byte by 1, when counting starting from the leftmost number)
Name the dumped files with a ".FNT" extension
Open the .FNT files in Square Enix Remakes Font Editor https://www.romhacking.net/utilities/624/
Make your changes, save, then insert them back into the same locations using the hex editor.
Hope that helps, sorry these instructions are sort of esoteric