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 not yet checked out repository subtrees. Select Set depth to working copy to get new subtrees as well (according to the Recurse into subdirectories option and the selected Depth).
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.
Use Rebuild SVN admin area completely to fetch all pristine copies and property values freshly from the repository. This option should only be used to recover from irreparable working copy defects (like 'checksum mismatches'). It will transfer about the same amount of data as a normal Check Out.