Just to help clarification, apparently you are attempting to update your existing operating system. An update downloads "fixes" to the existing OS. Apparently there are no updates as you are current. What you are attempting to do is to download and install an upgrade. An upgrade installs a new OS. The process for getting each one installed can be different. El Capitan should be available from the Apple App store or on the Apple support web site. Niel has provided you with a link for the download. One word of caution, do not use Wi Fi. It often corrupts the download and then it won't work. Use a wired ethernet connection to your modem/router. I hope it goes well.