If you want to stop working on a branch and move your local copy back to the main line of development, you have to make WinCvs remove all sticky tags. To remove the sticky tags, and thus update your local copy to the main development line, do the following:
Select the top level directory of the project. (If you know exactly what directories and files are part of the branch, you may select these instead.)
Click the right mouse button on the selction, and choose the Update selection menu item.
Make sure Create missing directories that exist in the repository is checked.
Click the Reset any sticky date/tag/'-k' options checkbox.
Press the OK button.
WinCvs will now update your local copy so it matches the current main line of development. The branch you were on still exists in the repository, and you may return to it as described in Selecting a Branch to Work on whenever you want to.