By default, the order of usage is (see documentation):
So if you choose to create your own versions of the base (CMS.resx) strings all you need to do is find the actual key name and enter that in the Localization app and enter your value. By default then, Kentico will serve up the database version since it was found vs. going to the file(s).
When you're done, depending on your cache settings, you may need to clear your cache.
Also check your config file for this key which would reverse the priority and make the database last priority:
<add key="CMSUseSQLResourceManagerAsPrimary" value="false" />