Google for "Download iPhone 4/4S iOS 5.0.1 IPSW" use a download manager to download it if possible. Save it on your desktop for easy find and use with iTunes. Once the IPSW downloading if done, connect your iPhone to your computer, open up iTunes, highlight your iPhone then go to summary tab and there you will see RESTORE. Before hitting on restore, press down and keep down "shift" key on your computer keyboard and hit restore. A dialogue box will then show up asking you to locate the IPSW file, navigate to your desktop and find the IPSW there, click next and follow onscreen instructions to finish the installation.