Updated: May 21, 2020
Disclaimer- This blog post/tutorial is strictly meant for educational purposes, please don't misuse it.
Files/Softwares/Tools you need to download-
You need to download the above mentioned files from the above mentioned links. The installation steps are quite simple 7 are clearly mentioned in the respective websites of specific tools.
After installing apktool on your system, open CMD as administrator & type in the command apktool, then hit enter.
To modify the source code of any app, you need the app in apk file format.
If you have the apk file of the app present in your system, you can just copy it & paste it on your dex2jar folder (dex2jar folder will be present in the Download section).
Then open CMD in the folder where you have downloaded the dex2jar & type in the command d2j-dex2jar.bat -f yourapp.apk & hit enter.
Using the above mentioned command, you can covert the apk file into jar file & you can read the source code using jd-gui.
But to edit the source code of the apk file & to modify it, you need to perform some additional steps. Open command prompt in dex2jar folder, type in the command apktool d yourapp.apk & hit enter, a folder will be created with the name of your apk file, in this case it's yourapp, you can go through the source code of the apk file in the folder & you can edit, change or modify the code according to your own wish using any text editor software like Notepad ++ or the jd-gui (Java Decompiler).
To recompile the app, open command prompt in the dex2jar folder & type in the following command apktool b yourapp & hit enter. The apk file will recompiled along with the changes you made to it, you can find it inside the yourapp folder in the "dist" folder.
We will also post a tutorial on this soon on our YouTube channel, stay tuned.
Thanks for reading! Have a nice day.