using Ms office Api in C# - Find & Replace

Hi All,
I have a doc or docx file. That file may contains, images, texts and some font formatting and some keys like <FirstName> and <LastName>.
if I call a function that function returns array of bytes read from the docx or doc file.
I have array of bytes. Now, I need to write a function to find the key <FirstName> and <LastName> and replace with some values.
This should be done by using MS Word API. Becuase, the file may contains images, text formatting. So, we need to do this without affecting the images and text formatting.
How to do this?

Thanks,
R. Eswaran