@Override
public void onBackPressed() {
    Intent data = new Intent();
    data.putExtra("Latitude", location.getLatitude() );
    data.putExtra("Longitude", location.getLongitude());
    setResult(RESULT_OK,data);
    super.onBackPressed();

}
 @Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if(requestCode == request_Code){
        if(resultCode == RESULT_OK){
                            Location location = new Location(provider);
            location.setLatitude(data.getDoubleExtra("Latitude", 0));
            location.setLongitude(data.getDoubleExtra("Longitude", 0));
            listener.onLocationChanged(location);
        }
    }
    super.onActivityResult(requestCode, resultCode, data);
}

@Override
protected void onResume() {
    super.onResume();
    // update your location here
}

Take a flag in Activity A.

you are going A->B
when you press back ,B finish();chack flag =true;and     you will come back to A,

Now in A's onresume() method check for flag and if flag is true then     update location