You can follow below steps.

1. Select File > Info.
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.

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

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