How to use svn merge to preview svn update
- Change to the root directory of your working copy.
- Run the command:
svn merge --dry-run -r BASE:HEAD .
Note: The trailing “dot” is the path of the working copy, and it is required even if you are already inside the working copy, which you should be after step 1.
- The above command requests a “dry run”
of what will happen if we receive all changes that occurred after our
working revision (BASE) and the most recent revision in the repository
(HEAD). This is what
svn update does, too.
- Review the output of
svn merge and check for conflicts. If you spot any, try using svn diff to determine what is causing the conflict, or just prepare yourself to fix the conflict ASAP! 
- Now that you know how friendly or destructive
update will be with your working copy, run svn update to let the fun begin!
SRC: http://translocator.ws/2005/10/12/svn-update-dry-run
