If you lose the password, you cannot open or gain access to the password-protected presentation. Microsoft cannot provide assistance for opening or modifying a presentation whose password you don't remember.
Open the presentation that you want to create a password for. On the Tools menu, click Options, and then click the Security tab. Do one of the following: To require a password for opening a presentation, type a password in the Password to open box, and then click OK. To require a password for modifying a presentation, type a password in the Password to modify box, and then click OK. Note Setting a modify password enables other users who do not know the password to view the presentation, but prevents them from making any changes to the presentation.
In the Confirm Password dialog box, type the password again, and then click OK.