I try to find a document listing the changes between UML 2.5.1 and UML 2.5, something like a release note.
I looked at the UL 2.5.1 document, look on OMG website, and bugs but I did not find a sum up of differences between the two versions.
There is version of the standard document containing change bars that highlight all changes compared to the previous version of the document. It is accessible on the OMG page https://www.omg.org/spec/UML/2.5.1/PDF/changebar