Login to https://lcs.dynamics.com/ with your CustomerSource or PartnerSource credentials.
Open the existing project, if you have no project then created a new project. For example I have Project. Click on it and open it.
From next screen. At button you find updates.
Next window you will find latest updates listed. For Example yesterday Dynamics Ax 2013 R3 Cu8 is released. Its updates appear at top. You can download it.