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.
For sparse working copies, the Update will not pull in files/directories of repository subtrees that haven't been checked out yet. Select Set depth to working copy to get new subtrees as well (according to the selected Depth option).
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 beforehand.
Use Include Externals to descend into externals.