It is set and FIXED that reset to the BACK button. It's the bottom button on the back. The issue is 2 fold. 1, every other camera I have, both Fuji or Sony, let me reset the focus point to center by pressing in the stick. So muscle memory has already been there for decade of use. 2, that button is totally flushed to the body, so it is almost impossible to press it when i want to by feel every single time. Whereas I know I have my thumb on the stick every single time.
I have looked into remapping it, and trust me, you can't. I even spoke to Fuji rep at the photography show and moaned about this to them a couple years ago. They could fix this with a firmware update but it's not going to happen now.