This is the Android version of Pachi, a Go game engine developed by Petr Baudis and Jean-Loup Gailly.
If you make a change to Pachi's code, you have to rebuild the executable with the Android NDK.
To do this, open the jni folder and enter the command ndk-build. If you get errors, try to use the NDK version r10e. Sometimes newer versions require to make some changes to the code. Then copy the executable from libs/armeabi to the app/src/main/res/raw folder and replace the existing one.
Pachi for Android is distributed under the GPLv2 license (see the COPYING file for details and full text of the license). You are welcome to tweak it as you wish and distribute it freely, but only together with the source code.