How to edit homepage in WordPress?

If it really is the header you want to edit, you can find its source here:

/wp-content/themes/Wallbase/header.php

Depending on A) what it is exactly that you want to customize, and B) how your theme is set up and what options it comes with, you might want to try the Theme Options, which can be found in the Appearance menu in WordPress Admin, if present (the options, that is).

If it is some CSS thing, however, you have to edit the theme’s stylesheet:

/wp-content/themes/Wallbase/style.css

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