Use Modify|Update to get the latest changes or a specific revision from the repository for the selected files/directory.
Select HEAD to get the latest changes. To get a revision, select Revision and enter the revision number. Select Recurse into subdirectories to perform the update command not only for the current selected directory, but also for all subdirectories.
When selecting Allow unversioned obstructions, SmartSVN will continue to update new files from the repository for which locally unversioned files already exist. Otherwise the update will be cancelled in such situations, giving you the chance to cleanup these locally unversioned files before.