We found a few issues and released 5.2.1 maintenance release today. Only the core files need updating if you have already upgraded to 5.2. If you are on a version below 5.2.0, please do the full upgrade routine.
A change log is available.
This fixes an issue we found with the smoothbox and also an issue with admin member level setting for user mention privacy.
Can I upgrade from version 5.1.0p1 directly to version 5.2.1?
Yes you can.