How to Modify or Change Android Apps .apk Files

Google uses .apk extension for Android Apps which might seem somewhat confusing but its nothing and just like a .zip file, so when you change the extension form .apk to .zip than application can be explored easily like applicationname.apk to applicationname.zip but still there are some problems than when you open this file you will see another classes.dex file which is not a normal file but really important and cannot be opened like anything you need a specific tool to open which is mentioned below.
Now classes.dex file contains every important thing + all the java based file but the true thing is that its encrypted so we are going to use the dex2jar opener given below to open that file up. 

1. Download Dex2Jar from http://code.google.com/p/dex2jar/
2. Download Java Decompiler from http://java.decompiler.free.fr/

Now after you have downloaded both above mentioned softwares you just need to copy the classes.dex file to the Dex2Jar extracted folder and run a small command from the commandline : "dex2jar.bat classes.dex"
Now this will create a file with a strange name something like "classes.dex.dex2jar.jar" so now you can open this using the WinRAR if you do not have it you can install it and start modifying your files.
[Via - AndroidGuys]