Quickly peeking over XDA I found this guide which is quite well explained for people who haven't decompiled an app before:
http://xdaforums.com/showthread.php?t=2341351
Steps 6-8 are unnecessary hence you're not modifying a System APP but is recommended anyway just for future use (and avoiding any problems - which shouldn't occur anyway).
Your app (assuming it's installed) should be under data/app (from root directory of the device, copy it from there to /sdcard/ and then to PC. (There are many file managers which have root access, ES File manager or Total Commander are recommended)
(Follow guide)
Once you decompile the app (will be under a folder called projects) there should be a AndroidManifest.xml in the root of the decompiled app folder, edit it (I recommend NotePad++ for many reasons, but WordPad/Notepad are still viable), the XML is in human readable format and is generally small so there should be no issues.
Somewhere there should be a package name (these 99% of the time start with com. and often include the developer name after it then the name of the app e.g. com.android.chrome or com.handlerexploit.tweedle), simply modify it to something different, save and go back to the guide to recompile.
After recompiling simply push the app back to the phone and install it through a file manager.
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---------- Post added at 01:06 PM ---------- Previous post was at 12:58 PM ----------
Also seeing you have not probably used ADB you'd need the drivers for it (as well as USB Debugging enables in phone) for APK multi tool to pull system files if you decide to do Steps 6-8 (although unnecessary).
The tool here :
http://www.wugfresh.com/nrt/ has easy to do instructions on installing drivers or if you know what you're doing grab the official driver here (
http://developer.android.com/sdk/win-usb.html ) and install in device manager if your device has no driver installed there. (Do Browse manually and point it to USB_driver directory)
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