this is in the office help, you should do this by you self

1. Open the file.


2. On the Tools menu, click Options, and then click Security.


3. Do one of the following:

Create a password to open

1. In the Password to open box, type a password, and then click OK.


2. In the Reenter password to open box, type the password again, and then click OK.

Create a password to modify