Are you asking about:
- a Numbers document? If you have never saved the document then select the menu item "File > Save…" . If you want to make a copy of a document that has already been saved AND the document is open select the menu item "File > Duplicate"
- a sheet? Double click the sheet name in the sheets summary list
- a table? Double click name of the table in the sheets summary list
Depends what you mean when you say "spreadsheet".
If you're using the word in the Numbers sense to be a synonym for the Document or the File, you rename it the way you would rename any file:
- Close the document.
- Go to the Finder.
- Locate and select the File.
- a) Click on the file's name, pause, click again to highlight the name.
Type to replace the name OR
Click at the point in the name you wish to make a change, then edit the name.
b) Click the file's icon, then press command-I to Get Info
In the info window:
(If necessary) click the triangle to open the Name and Extension section
Click in the Name box.
Edit or replace the name.
- Close the Info window.
If you mean 'How do I rename a Table in a Numbers document?' the process is similar. Click on the Table's name in the Sheets list, then follow the steps in 4a above.