From http://cheatsheet.logicalwebhost.com/mikrotik/
DdeC 6 Apr 21


Power off unit FOR 30 SECONDS.

Hold paper clip in hole on back panel depressing switch while plugging in the power cord. 

Keep it held in for around 5 seconds while it boots and the USER LED starts flashing. 
Now release the button to clear configuration. 


Note: If you wait until LED stops flashing, and only then release the button – this will instead launch Netinstall mode, to reinstall RouterOS.