As modifying android app is nor easy neither difficult task.
Here modifying seems reverse engineering or reengineering.
This content is only for the educational purpose not for any malicious activities.
You can see android app has .apk extension.You can extract this apk file like zip files but what you get is only compiled class file you can’t simply deal with them.
So here comes a tool named apktool, which is used to decompile, rebuild and install frameworks.It decompiles nearly to the source code.
Apktool decompiles only XML and png files for re-engineering.To get java code there two tools plays their role named as dex2jar and jd-gui.
Dex2jar, this tool converts .dex files into jar java packages, this is further used for editing java codes.
Then these jar files can be converted into java file with the help of jd-GUI software.
These tools were available for Windows,Linux and Mac platforms.
A brief tutorial video will be posted in our you tube channel very soon. Stay tuned.
ALSO READ: how to extract android source code
