You can't.

Concrete5, as most of the CMS out there, is built upon a database pulling in content to a view layer which structures and layouts the content.

Concrete5 uses page types to display the data and depending on the page requested a different (or the same) page type is used.

If you want to change the layout (HTML, CSS...) you need to locate your sites theme files and change the structure in the page types.

If it is a core theme you will find the theme files in;
'root/concrete/themes'.

If you are using a downloaded theme you will find the files in;
'root/packages/[theme_package_name]/'.

The third place you might find a theme is where your own developed theme might reside (if not created and installed as a package);
'root/themes'.

If you want to edit any of the themes you should first create a new theme from the original so you don't loose any of your edits on an update. Check out these links for more information:
http://www.concrete5.org/documentation/how-tos/editors/customize-a-...
http://www.concrete5.org/documentation/how-tos/designers/modify-a-p...
http://www.concrete5.org/documentation/how-tos/developers/change-th...
http://www.concrete5.org/documentation/how-tos/designers/making-a-t...

You will find a lot of information in the Documentation section regarding themes and theme development. Also the forums are great resources for common problems. Just use the Search function and you will probably find what you are looking for.