In a working copy of the branch, merge the revision that updated A.cpp. Alternatively, if you have no local branch changes you need to preserve, you could just svn copy A.cpp from trunk and overwrite your local A.cpp.

Example

cd workingCopyOfX svn merge -r51 http://my.repo.com/trunk/A.cpp 

assuming the fix was made in r51

In a working copy of the branch, merge the revision that updated A.cpp. Alternatively, if you have no local branch changes you need to preserve, you could just svn copy A.cpp from trunk and overwrite your local A.cpp.

Example

cd workingCopyOfX svn merge -r50:51 http://my.repo.com/trunk/A.cpp 

assuming the fix was made in r51