Once created and saved either in the main Numbers root, or in a sub folder, the Numbers Spreadsheet can be moved around using the usual drag and drop system.
If the spreadsheet is in iCloud Numbers, or a folder with sub folders, then it can be dragged easily onto a folder.
If the spreadsheet is in a sub folder and it needs to be moved back towards the main root then the folder path at the bottom of the window will need to be used. Drag and drop the spreadsheet onto any of the folders listed to move the spreadsheet.