How to Link Header to Another Site

1.Log in to your dashboard--> layout- -> Edit HTML

2.Click on "Expand Widget Template"

3.Scroll down to where you see this:-

  <div class='titlewrapper'>
  <h1 class='title'>
  <b:if cond='data:blog.url == data:blog.homepageUrl'>
  <data:title/>
  <b:else/>
  <a expr:href='data:blog.homepageUrl'><data:title/></a>
  </b:if>
  </h1>
  </div>

4.Replace above code with below code. 

  <div class='titlewrapper'>
  <h1 class='title'>
  <a expr:href='LINK_OF_YOUR_ANOTHER_SITE'><data:title/></a>
  </h1>
  </div>

NOTE : Replace 'LINK_OF_YOUR_ANOTHER_SITE' with actual URL of your another site and save template.