See the Wolfram documentation for NotebookWrite.

Carl's suggestion on NotebookWrite can well be used. Since data can also be a complete notebook. An example:

Save the following notebook to the default directory ($HomeDirectory) and give it the name Example.nb

Then, evaluate this in another notebook:

NotebookWrite[EvaluationNotebook[], Import["Example.nb"]]

I think there may be confusion about what a notebook window is. The comparison with the documentation is unhelpful because the documentation notebooks have a key characteristic: you can edit them, but they don't save. So it is straightforward to refresh their content. Regular notebooks are not like that: they have a name which links them to the saved file. So what do you want to happen? book1.nb is open with the name book1.nb, linked back to the file book1.nb. When you open book2.nb, do you want the window name to change to book2.nb? I'm guessing you do. But that is a new window. What you are really asking is for a new window to appear in the exact same location as the previous window, and for the previous window to go away. Which maybe you can do programmatically. But what you can't do is just 'replace the contents of the window with a new notebook'. You can replace the contents of the window with the contents of the new notebook, as others have demonstrated, but it will still be book1.nb.

So you want the current window opened with book1.nb to be entirely refreshed with contents of book2.nb and be named book2.nb. Seems like feasible but why?