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.

Warning: 

  • Microsoft cannot retrieve forgotten passwords, so be sure that your password is especially memorable.

  • There are no restrictions on the passwords you use with regards to length, characters or numbers, but passwords are case-sensitive.

  • It’s not always secure to distribute password-protected files that contain sensitive information such as credit card numbers.

  • Be cautious when sharing files or passwords with other users. You still run the risk of passwords them falling into the hands of unintended users. Remember that locking a file with a password does not necessarily protect your file from malicious intent.


Source: Microsoft