This is the shortest way, works both ways Asus<->Merlin:
- reboot the router to free the memory
- umnount USB drives after it boots up (if any)
- flash the new firmware on top
- let the router boot, don't log-in, watch the LEDs
- power it off, hold the WPS and power it back on
- let the router boot, set it up manually
I always flash those on Wireless. Like many Asus customers do. Just press the button. No chance I'm going to the router with my laptop for no reason. If the router wants to come to me, let it come. If something goes wrong, just repeat. Like going from 81930 to 81992 on RT-AC86U. JFFS partition went South after this update. WPS and Power button reset and back in business. If something goes terribly wrong,
@laracroftonline procedure applies. Never had to do it though. Only when jumping between Asuswrt, Tomato, DD-WRT on supported routers.