Use Modify|Move to move and/or rename a file or directory which is already under SVN control. The file with the old name will be scheduled for removal, the file with the new name for addition. This command will preserve the history of the moved item.
There is also a special mode of this command that can be used to tell SmartSVN 'after the fact' that a file was moved. For this to work, exactly two files must be selected: One that is missing or removed, and another that is unversioned, added or replaced. SmartSVN will then remove the missing file (if necessary), add the unversioned file (if necessary), and connect the history of the added file to that of the removed file.
You can also use Drag-And-Drop to copy or move files and directories.