how to insert webmastertools code ?

Hi,

Not sure what exactly you're trying to add, but here is how to add CSS/PHP without having to edit the core theme.

Adding CSS - GeneratePress
Adding PHP - GeneratePress

If you're adding Javascript to header, you might use GP Hooks Add-On if you've purchased. I personally use that to add all those extra Javascript codes like Analytics, AdSense etc. Beauty of this Add-On is that you can add the script to almost any section in the webpage.

Not using any Add-Ons? You can create a child theme so that you don't lose your customizations when a theme update is released. Here is how to do that.

How do you make a child theme? - GeneratePress

Thank you so muchhhhh

You're very welcome!

-Ramesh
Just a volunteer here.