Substituting a page in multi-page document in Pages

I have a 50 page document in Pages. I opened a separate Pages document and revised and retyped page 29. I have both my 50 page document and my single page 29 document on my desktop. I want to substitute my revised page 29 into my 50 page document and delete the original page 29 there. I tried dragging my page 29 document from the desktop into the thumbnails of the 50 page document but it keeps bouncing back. You assistance would be appreciated but please be specific on all required steps that I need to take. Thanks ever so much