In an ideal world, the header of your website isn't something you want to be changing regularly. It's a critical bit of navigation for your users, and it's been designed by our team to not really be fiddled around with.
That's not to say nothing can ever change - not at all! We just advise to proceed with caution, and ideally get in touch with your dedicated website manager to discuss any changes you want to look at doing. It's often best that we action them for you.
If there's specific changes you want to make, get in touch with us and we'll show you where to go in the admin area so you can control it yourself.
Just so you know, the header can't be edited in sandbox mode. The only way to edit it in canvas mode is to edit the live version :)