Since the application is free of charge, I suggest you upgrade it to the latest version. If you use Windows XP or below, go to Start > Control Panel > Add or Remove Programs. In the Currently Installed Programs box, click on the 4shared Desktop program and then click Remove.
If you have Windows Vista or above, you need to go to Start > Control Panel > Programs > Programs and Features. Select the 4shared Desktop application and click on Uninstall.
After that, go to the official web page, click the Download Now! button, save the file to your computer and install it following the self-explained instructions. Also, get sure that your Windows firewall and antivirus program doesn't block the software.