There are multiple ways to get back from the running-configuration to the startup-configuration on a Cisco router. One of the simplest ways is just rebooting the router, but this takes a couple of minutes. You can also issue a copy startup-config running-config, but that doesn’t actually replace the configuration, but merges both together.
A very powerful command to revert to the startup-configuration is:
configure replace nvram:startup-config
This command saves you some time because a reboot is not required.