So, you've made a change in one branch, and you need to make it in the other. You could edit the files twice, but that isn't very efficient. Instead, you could get CVS to merge one branch onto another.
To merge the changes in foo.c in your Stella_1.0 branch into the version on the head, go to where you have the head-branch version checked out into your sandbox, then type:
cvs update -j Stella_1.0 foo.c
Sometimes, the changes you made in one branch conflict with those you made in another. If this happens, CVS displays a C before the filename when you merge the versions. CVS leaves both versions of the conflicting lines in place, but marks them with rows of greater-than, equals, and less-than signs. You should edit the file to correct the discrepancies, and then check the corrected version into CVS.