Problems upgrading to El Capitan from Yosemite.

I have problems trying to upgrade from OS X Yosemite to OS X El Capitan. Soon as I try to update from iTunes it stays in pause mode. Once I restart the computer I go back to iTunes and the button says resume, I click it then prompts me to sign in and then goes back to pause mode.

My version is 10.10.5

(Macbook 13.3 inch June 2010)