Go into enable mode: en
Configure terminal: config t
Set to factory default: config factory-default
Hit the space bar at each of the “more” prompts.
Now write to memory: wr
Confirm with: show start

If you see an error:
ERROR: command can only be executed in single router mode

Erase the startup config: wr erase
Then reload (Don’t save current config): reload

cisco-factory-default