Redirect home page in simple html hyperlink

I know its very funny question.
But I am not able to do this even after using google.
 
I am developing a web project in simple html.
Site name is xyz for example contains index.html page.
under xyz folder I have created a abc folder contains index.html.
 
I can went to abc from xyz.
by how to access the xyz\index.html from abc.
 
go back to parent directory from sub directory?