You can't modify apk's without signing them again. The Android system will notice that the apk has been tampered with and refuse to load it. You could sign it with your own key, but then your signature wont match the official signature for that package name and that will prevent you from getting any updates for that application as long as your modified apk is installed. Also if that application is relying on being signed with the same certificates as other applications from the same manufacturer it wont work correctly.
You can read about signing android applications in the official documentation
Why do you want to do this?