If you want to just manually keep different configuration files on the switch for testing, @harriuljas has got your answer, though I would activate the new configuration by using "copy flash:/somefilename startup-config" and rebooting instead. Copying a configuration directly into the running-config merges the old configuration with the new one, which may not be what you want.
If you want the switch to keep an archive of the last configuration changes, so that you can roll back when you're making changes, there's an even easier way to do that.
do mkdir flash:/startup-config
!
archive
path flash:/startup-config/
maximum 8
write-memory
This will automatically save a copy of the configuration to the flash:/startup-config directory every time the "copy running-config startup-config" command (or its equivalents such as "wr" or "write memory") is issued and will keep up to eight previous configurations, making it very easy to roll back to one of the previous ones.
