Unable to change class and id names in header.php

In header.php in my child theme I am trying to change the following class and id names

<header id="masthead" class="site-header" role="banner"> 

into

<header class="header" id="myTopnav" role="banner">

I changed class and id names manually but ‘view source’ still showing original ones - id=“masthead” class=“site-header”

Are those WP generated classe and ID? Is there a way to change them?