Open up a command prompt for the user that you have vnc server running as.
Then type

vncpasswd

You will then be prompted to enter a new password twice. Choose your new vnc server password and then restart the vncserver service using the following command.

service vncserver restart

The above commands work when run as the root user on Centos but should work similarly on most other variation of Linux.

1. On the File menu, click Save As

2. On the Tools menu, click General Options.

3. Do either or both of the following:

1. • If you want users to enter a password before they can view the workbook, type a password in the Password to open box, and then click OK.
   • If you want users to enter a password before they can save changes to the workbook, type a password in the Password to modify box.

 Note   Unlike passwords you specify in the Password to open box, passwords you specify in the Password to modify box are not encrypted. These passwords are only meant to give specific users permission to modify workbook data. For optimal password security, it’s best to assign both passwords. An encrypted password to access the workbook, and one to provide specific users with permission to modify its content.

 Important   Use strong passwords that combine uppercase and lowercase letters, numbers, and symbols. Weak passwords don’t mix these elements. Strong password: Y6dh!et5. Weak password: House27. Use a strong password that you can remember so that you don’t have to write it down.

4. If you want to use a different encryption type, click Advanced, click the type you want in the Choose an encryption type list, and then click OK.

5. If needed, specify the number of characters you want in the Choose a key length box.

 Note   Document property encryption is enabled by default for most encryption types and providers. It prevents unauthorized users from viewing summary and custom file properties (such as the author or any custom file information) in the Properties dialog box. When users right-click the password-protected file, and then click Properties, information won’t be available on the Summary tab and Custom tab. Authorized users, however, can open the file and view all file properties (File menu, Properties command). To disable document property encryption, clear the Encrypt document properties check box.

6. Click OK.

7. When prompted, retype your passwords to confirm them.

8. Click Save.

9. If prompted, click Yes to replace the existing workbook.

 Note   You can also secure a workbook with a password on the Security tab of the Options dialog box (Tools menu, Options command).