Current MikroTik RouterBOARDs do not have a reset button (if you find a button on your RouterBOARD, it’s not the reset-to-factory-default button) to get it reset to the default factory configuration. User guides for each RouterBOARD, which MikroTik provides on routerboard.com, define the identifier and location of the reset mechanism. Hint: look for a distinct hole on the RouterBOARD and short-circuit it.
I will post some pictures when I get the time chance.
To reset RouterOS to the default configuration, execute the command /system reset-configuration. That should do the job. It will do a backup of the current configuration prior to the reset (how convenient!), in case you change your mind and possibly prevent accidental resets.