Pretty much the only program that can open and work with all versions of .numbers spreadsheet files is Numbers spreadsheet app for Mac, formerly part of iWorks office productivity bundles. Windows users usually struggle with .numbers files and prefer MS Excel or OpenOffice.org spreadsheets.
If you do not own Mac with Numbers and no friend of yours has it, the most convenient way to achieve numbers to xls conversion would be to make an iCloud account and use the online based iWorks apps. Except for that, some latest version of LibreOffice might support some .numbers spreadsheets, so this is worth a shot as well.Open NUMBERS file Open XLS file
Full-featured spreadsheet tool from LibreOffice suite
A preadsheet editing application for iOS
A spreadsheet application from Apple