In the patch set notes you can read what changed.
If you're still on APEX 4.x you can go immediately to APEX 5.0.1, you only need to download the latest version on OTN.
If you're already on APEX 5.0, you can download the patch set from support.oracle.com, search for patch number 21364820. Applying the patch took less than 5 minutes in my environment.
This patch set updates the Universal Theme too, so don't forget to update your images folder. When you login in APEX after the patch, it will check if you have the correct images folder, if not it will give you an alert. Although I updated the images directory I still got that alert due to browser caching. Refresh your page and it should be ok.
Note that it's important to be on APEX 5.0.1 when you use APEX Office Print - currently available in beta for a select audience, public release end of this month (July 2015). Behind the scenes we use the APEX_JSON and APEX_WEB_SERVICE packages which got an update in APEX 5.0.1.
And finally, there's a nice new D3 chart available in APEX 5.0.1 called "D3 Collapsible Treemap Chart"
Happy upgrading...
Update 18-JUL-2015: there're still a couple of known issues, please read those too and install some patchset exception when necessary.