By the rescue mode : Same method for all models
First you need the Restoration Tool from asus(asus site
https://www.asus.com/us/Networking/RTN66U/HelpDesk_Download/) , install it
after you need to put the router in RESCUE MODE
Follow these steps its for Ac66(but work for all models). You need to do all the steps from the link below
https://www.asus.com/support/faq/1000814/
*How to enter in RESCUE mode:
Power off the router: Hold RESET button and power on router,keep holding RESET til led begin flashing slowy, release RESET button and you are now in rescue mode
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This is the last time i post the steps for downgrading 
.Peoples dont read **