Next PC must be incremented. It would be possible to make PC be a "super register," one that has its own increment wire. But a more uniform, albeit slower, approach would be to add the +1 register to PC and store the value back into PC. This requires inputting 2 to the A decoder, 1 to the B decoder, selecting F=add, setting result-LD to 1 and then, after a suitable delay, selecting PC as the destination by putting 2 on the C decoder inputs.