This article provides information on which is more convenient and cost-effective: whether to republish the app or whether to respond to Apple's rejection email after addressing observations that Apple mentioned.
If the changes made to the app were only in the Build or Design sections, there is no need for you to start the republishing process from within BiznessApps. If that is the case, you can simply respond back to Apple letting them know that you have addressed their concerns and they will review the app again. This is the fastest method and it doesn't have any fees involved.
You can also republish the app on BiznessApps (required if you made any changes in the Publish section of the app) and there are also no fees for this but the process will take longer as our iOS Uploads Team will have to review the app again.