Im not quite sure what you mean by "Download Excel ... using open office". You would normally download anything from the Internet. I believe Excel is available from the Microsoft website.
Open Office is a suite of programs which does include a spreadsheet program.
Do you mean save the spreadsheets in Excel format? if this is the case, then go to (i think it is in) the edit menu, choose "Save As..." and when there give the spreadsheet a name, choose where you want to save the document, and in the other drop down menu (underneath the file name field where you give the file a name) click on the down arrow on the right of the box and choose Excel in the long list that appears. Press OK and it should be saved in Excel format.