Start with KB2919442, the March 2014 Maintenance Stack Update for Windows 8.1 and Windows Server 2012 R2.
Then you need to install KB2919355, the update of April 2014, which is a package consisting of six separate updates and an update tool. These must be installed in the following order:
You can then install the latest update of the maintenance stack, as shown in ADV990001.
Finally, install the latest monthly update rollup, as shown in KB4009470 – check the list of updates in the left margin; the last total should be at the top of the list.