Use the Save As command to rename and save a Terrain with another name. Save As preserves the last saved version of the file, and saves a copy under the name you give it.
You can also use the File - Save As command to export the file into other file formats. See the Export Setup Dialog for a complete list of file formats available, and instructions on how to add and remove file formats.
Push the "+" button next to the "Files of Type" combo box to popup the Export Setup Dialog. This will show a complete list of file formats available, and allow you to add and remove file formats.