does anyone have a step-by-step guide to changing the transmission fluid in a 2015 Q50 RWD? I have the online factory service manual, but it seems a little complicated/technical to follow..

That's the procedure.

The other option is simply to drop the pan (assuming there's no drain plug) and reinstall with a fresh gasket, then refill. This won't get ALL the ATF but about 60%.

If you want a full flush, any reputable shop should have a trans flush machine (or the dealer can do it).

I know i'm a bit late here but the Q50 transmission is a sealed unit, so it's not exactly an easy transmission to service. There is no accessible filter either. Honestly i've weighed out doing it myself compared to what the dealer charges and it's not really worth it, at least for what my local dealer charges for the service. Considering how finicky Nissan/Infiniti transmissions are and how picky they are about the fluid I wouldn't take the risk.

The 7sp fluid is supposed to last a lifetime and not need a flush, thus, why it's a sealed system.