With APK Editor you can choose between two types of editing: FULL EDIT, which lets you rebuild files from an APK; and SIMPLE EDIT, which lets you replace files inside an APK. The first is a much more complex and strenuous process, while the second type of editing can be done much more easily.
Regardless of which type you choose, you'll quickly realize that it's extremely easy to edit any aspect of an APK. You can change an app's background image, add or remove languages, or even delete permissions. Everything depends on how skilled you are.
APK Editor is a ... tricky app. When used well it can bring you lots of joy, but if used poorly... not so much. In any case, you can find some examples and advice for using the app correctly on the app's help page.
Requires Android 3.1 or higher