Does anyone know how to change the AFR table on Mega tune from 64 cells (8x8) to 144 cells (12x12) please?

You can't, that's all that can fit into the available space on the processor. TBH I don't see why you'd need 12x12 on the afr tables anyway, other than it 'being nice' to have the bins at the same places as your fuel table. I've found that I struggle to fill an 8x8 AFR table, it's not a great idea to use closed loop control for the whole operating range of the engine so it's covering a smaller range than the other tables.