Microsoft will officially release Windows 8.1 preview later today at the BUILD conference in San Francisco. Below is a link that will allow you to download the ISO for the Windows 8.1 Preview. Alternatively, you update Windows 8 through the Windows Store. The link is not live yet, but will be later today.
Download Windows 8.1: http://prev.windows.microsoft.com/en-001/windows-8/preview-download