There is a software package that was created for this type of situations when users are blocked from accessing certain websites. Please keep in mind that using this software could violate the laws of your country and since you are in Iran, it's very dangerous.
If you proceed to do this at your own risk, then please download and install Hotspot Shield as it's a free product and offers 100 % anonymity for you connections and bypasses filters imposed by the ISP. You can access and download everything you want without any restriction.
If the packages asks for driver installation, accept the installation and perform a computer restart if necessary. Use the icon on your desktop to initiate the connection problem and then you can browse the web safely.