I Cannot update Windows 8 to Windows 8.1

I have Windows 8 installed on my Dell Inspiron 4050 laptop so I wanted to upgrade it to Windows 8.1 and I downloaded ISO from the Microsoft website and when I mount the ISO file it gives me the error file corrupted. Then I again tried mounting with both PowerISO and UltraISO too. I have re downloaded the file but no luck for me. Can anybody help me?
