Question [Q] Can't modify .apk files, Also can't see them on my PC.

For fun I made custom textures for 'Flappy Bird'
When I try to move the new texture file into the .apk it doesn't work and it disappears after I load the .apk back up.
I am using root browser, I basically can't do anything with the files in the .apk besides extract or view.

Also when I tried to modify it with my PC It wasn't even there, I tried and tried by making files and changing the extension but it wouldn't appear.

How can I fix these problems?
I really want to modify the texture for 'Flappy Bird'