Hi Candice.  I went into the template section of my blog and click on the customize button not edit html.  You would only click on edit html is you downloaded a free template and you want to upload it to your blog.  

After you click to customize you can go to adjust widths of your template and it should allow you to stretch your header and adjust it to your liking.  The only way that you would not be able to do this is if you have a premade ready template that is fixed.  You will not be able to adjust it.  I hope this helps Candice.  If not, let me know and I will see what I can find out.