How to tell strings.Replace() where to write the new string?

Question

go

How can I force the strings.Replace() function to write a new string to a specific location in the text of a file?
Now she writes to me at the end of the content.
As far as I understand, you need to specify the place in the text where the replaced string begins ... It's better to do this using ReadSeeker, or maybe FindString()?
And how to do it?