In my quick check of the spreadsheet, it appears that headings and default values are "locked" and would require a password to change or delete. Normally, you wouldn't change these, so no big deal. The author was trying to keep you from inadvertently changing something that shouldn't be changed.
Values that you would change to suit your personal situation appear unlocked where I checked. So, change away.
If you really want to customize any sheet, e.g., to format it differently, you can always copy the entire sheet and paste into a new one. At that point, you can do what you want with any cell. It's a pretty simple excel task that I won't go into here.