A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Press F12 to display the Save As dialog.
Click the Tools dropdown to the left of the Save button, and select General Options... from the dropdown menu.
Leave the 'Password to open' box blank.
Enter a password in the 'Password to modify' box.
Click OK. You'll have to re-enter the password as confirmation. Do not forget it!
Now save the workbook; it's OK to overwrite the existing version.
When users open the workbook, they'll be prompted for the password. They can click the Read-Only button to open the workbook without providing the password.