In the last part of chapter 1 it was introduced that you could remove the gloss effect on the app icon, however for this to take effect a rebuild is not enough, you need to do a clean (Build -> Clean) and then build and run it again.
“Cleaning” a project is just a hack fix for Xcode’s buggy build system. Occasionally, Xcode will make a mistake and not realize a change in your project’s bundled resources and consequently won’t copy the changed file into the build product (your application). The only way to fix this is to just trash the entire build product and rebuild, which is exactly what a clean is.
This shouldn’t happen every time you change a resource (image, Info.plist, xib file), but as you found out, it can happen, and clean is the fix in those somewhat rare occurrences.