Link to a different page within an included file, to be loaded on the same parent file

Link to a different page within an included file, to be loaded on the same
parent file

I hope this question makes sense because it seems simple, yet I can't work
out exactly how to word it in an understandable way.
Basically - I have an Ajax system for loading content onto an HTML file
via a seperate PHP file. This is to not need the actual page the browser
has loaded to change, only the content.
What I would like to do it be able to create a link WITHIN one of the
loaded content pages, which will change the content to another page via
the parent. For example:
Load the 'menu' page using Ajax, onto a div contained in the main page
Click a link on the 'menu' page, and a different page is loaded onto the
same div on the main page
Cheers