how to edit button left from menu

I can't find where to edit the "donate know" button. (at screenshot)

It's a custom HTML module in header-cta position, you can edit in backend of your site:

Here is the default content output of this module:

<div class="m-0"><a class="btn btn-success" href="index.php"> Donate now </a></div>

If you don't see this code, you need to turn off the default editor in Global Configuration: