How to set RB800 Factory Default?
http://wiki.mikrotik.com/wiki/Manual:Configuration_Management
Command name: /system reset-configuration
If I cannot access to RB800 with winbox, how can I do?
Setting is mixed, so I cannot access to IP address.
Can I use Reset Button? how?
Or can I use Serial port to input command?
yes, this command can be used in CLI no matter how you open it. Also, in user manual you can find reset hole that will wipe the configuration from the device. But that is - if you have physical access to the router (similar to serial console cable)