To decompiling and android APK file you require to use a tool name APKTool.

Apktool - A tool for reverse engineering 3rd party, closed, binary Android apps.

To decompile follow these commands:

apktool d <APK Name> 

To recompile:

apktool b <APP PATH>

Look at some important switch of this tool:

• -r or --no-res
  • With this switch apktool does not decompile everything is in resources.arsc and AndroidManifest.xml files.
• --force-manifest
  • Decode the APK's compiled manifest, even if decoding of resources is set to "false".
• -s or --no-src
  • With this switch apktool does not decompile classes.dex files.

How to get already installed APKs?

How Get APK Files Downloaded From Play Store