There are two main configs with Cisco IOS:

startup
and
running

When making changes in en(able) mode, these changes are made to the 'running' config.
If things go wrong, simply reset the router and it will boot to the last saved 'startup' config.

ONLY wr(ite) changes whilst in en(able) mode when happy it works!

To COPY one config to the other:

copy start run - copies the startup to a running config
copy run start - copies the running config to a startup config.
(This is the same as wr(ite) changes whilst in en(able) mode)

Most routers have enough memory to store 'test' configs'.
You can also backup your startup config to another file with:
copy start 'newbackupfilename'
to restore a damaged startup config simply;
copy 'newbackupfilename' start

Also:
copy run test1
saves current running config as a test1 config
(remember you're only ever working with running-config).