Summarize the following:
If you aren't signed in to your Google account, follow the on-screen instructions to do so now. It's at the top-left corner of Sheets. Another menu will expand. If you want to be able to edit the file as a spreadsheet in the future, it's a good idea to select Microsoft Excel (.xlsx). This is the folder to which you'll be saving the file. The Sheets file will now download to the selected folder in the selected file format.
Navigate to https://sheets.google.com in a web browser. Click the sheet you want to save. Click the File menu. Click Download as. Select a file format. Navigate to the destination folder. Click Save.