Assume the following scenario.
I have two files A.cpp and B.cpp in trunk. At revision 50 I create an experimental branch called X. Therefore at revision 50 both trunk and branch X are the same.
I continue to work on trunk and add C.cpp and D.cpp. I then realize that there is a bug in A.cpp and so I fix the bug.
Now my question is how do I send the updated A.cpp to branch X without sending the other files(B.cpp C.cpp D.cpp)?
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 copyA.cpp from trunk and overwrite your local A.cpp.Example
assuming the fix was made in r51