This is simple tutorial but will be helpful for any Google Docs beginners. If you want to rename an already saved Google Docs Spreadsheet, navigate to document list and select the spreadsheet you want to rename. if you have have got lot of documents then you can try filtering out only spreadsheets using the option provided as part of the Documents menu
After selecting the spreadsheet from the listed documents, right click on the spreadsheet and click Rename from the list of available of menus.
In the Rename dialog box, provide the required name and click OK to confirm the changes.