to clarify what SG stated. When you open an existing excel file with Numbers, Number imports the excel file into a new , temporary, Numbers document. The .numbers file does not formally exist on you machine until you save the document. You could try to locate the temporary files if you want.
There is no Number way to open/modify/save directly and excel document
The Numbers process is:
excel document -> import into Numbers into a temporary document
modify as needed
export from Numbers back to excel
if you want to retain the changes, save as a Numbers file.
if you modify the contents of the new document, you can export an updated excel file (or maybe save over the previous file). The whole process for collaborating with excel users in Numbers is really not great. IF you dod this often, I suggest working in excel