Well, sorry, but you did not answer the most important question. However, possibly you do not know the answer.
A Header is a section of Word document that is NOT in the main text body. Generally speaking users do not go in there, as it takes a few wriggles to get into the header.
This is normally the place that business (or people) put "letterhead". It takes it out of the normal editing.
So, you have two choices.
1. You could put the name / address etc in the Header. Put this IN the Header.
COMPANY NAME AND LOGO
Address
City, State Zip Code
Then the user will just enter text normally, as normally they are not in the header. This is why I asked if it was in the Header - the normal place for such things. But again, perhaps you don't know if it is a header or not. It does not matter. You put things in the Header by going View > Header and Footer, and then put in your name / address etc.
2. The suggestion from mintjulep. This is to use Section breaks to separate the name / address etc from the rest of the text body. Put the name / address etc in. Go Insert > Break and choose Continuous section break. This puts a section break. Now you have TWO sections. Go Tools > Protect document, and select Forms. Select the Sections button, and uncheck Section 2. You can use a password, or not...up to you. If it is password protected the user will not be able to change anything.
Once the document (section) is protected, NO edits are permitted. So if the Section with your name / address IS protected, the user can change anything. The rest of document they can do anything they want to.
Using a Header for this is probably the easier, and is certainly the most common practice.
Gerry