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.