Question: How do I edit an manifest file?

Answer: 
Unless you have permission from the developer of the APK to do this, you will be committing a copyright violation. This in turn may lead to your application being banned from the Android Market, if the developer of the APK complains to Google.

I encourage you to work with the developer of the APK to license the source code, so that you can use it properly. Or, integrate via `Intents` and help the user download the APK from the Android Market if you detect via `PackageManager` that the APK is not on the user's device.
Just because you've bought one copy of some developer's application, it doesn't entitle you to rip off their code and resell it multiple times in your application. That would be theft of intellectual property rights.