Redirect home page in simple html hyperlink

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HTML

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?