How do you restore a device to its previous configuration or factory defaults?

Undo unsaved changes by reload (reverts to startup-config) or removing commands; for factory defaults run erase startup-config then reload.

To discard unsaved changes:

• Remove changed commands individually, OR
• reload - Reload the device (restores startup-config)

To restore factory defaults:

Switch# erase startup-config
Switch# reload

Warning: erase startup-config deletes the saved configuration. After reload, the device starts with no configuration (factory default).

Go deeper:

• Cisco IOS (Wikipedia) — the boot and configuration-file model that reload and erase startup-config act on.