Essentially, it will change the name of the app as it is installed.
For example, I would like the OI File Manager App to appear simply as File Manager In my phone.
Changing the name of the file will do nothing. When the app is installed, it'll still have the same name (file name of the APK doesn't matter at all).
It really is easy.
Use this GUI Program to Browse for the Original APK file, and click Decompile.
Next, Click the "Navigate to Decompiled Directory", and it will open a folder containing the decompiled files.
open the AndroidManifest.xml file using Notepad++.
Its free, get it here: http://notepad-plus-plus.org/download/v6.2.3.html
look for something like the following (in this example, I am using a manifest from Canabalt):
<application android:label="@string/app_name" android:icon="@drawable/icon" android:name=".CanabaltApplication" android:debuggable="false">
Sometimes it will say something like android:label="Canabalt HD", in which case you'd just change it to what you want, and then rebuild it, but in this case there is an @ at the start of the name, which means that the name is stored in another file. As it is "@string/app_name", this means that the name is stored in the file "res\values\strings.xml" and has the name "app_name", so go into the folder "res", and then within that the folder "values" [if you use another language on your phone than the app's default language, go into the relevant folder e.g. for Spanish, you would go into "values-es" instead] and then within that open the file "strings.xml". Then you would look for something like this:
<string name="app_name">Canabalt HD</string>
Use the GUI program or zipsigner 2.
Also note that if there is an @ in the AndroidManifest.xml, you can still just edit the title in that file without finding the relevant entry in another file. In my above example, if I wanted to change the name to "Canabalt MOD", I'd change it to the following:
<application android:label="Canabalt MOD" android:icon="@drawable/icon" android:name=".CanabaltApplication" android:debuggable="false">
Install the app and youre done!
Credits to SifJar
btw This is how i modified my first app