How to Protect an Excel file

To prevent others from accessing data in your Excel files, protect your Excel file with a password.

2. Select the Protect Workbook box and choose Encrypt with Password.

3. Enter a password in the Password box, and then select OK.

4. Confirm the password in the Reenter Password box, and then select OK.

Ref: https://support.microsoft.com/en-us/office/protect-an-excel-file-7359d4ae-7213-4ac2-b058-f75e9311b599