Log into the admin panel of your store. Navigate to CMS> Pages> Home:

Select Design tab. You will find banner reference there:

<block type="cms/block"  name="banner_block">
<action  method="setBlockId"><block_id>banner_block</block_id>
</action> 
</block>

This means banners are controlled by a static block.

Go to CMS> Static blocks> Banner Block:

Search for the following piece of code:

<div  class="banner">  
<a href="{{ store  url='page_url.here'}}">
<img  alt="" src="http://osworlds.com/skin/frontend/base/default/images/media/banner_image.jpg"/>
<h>Banner title here</h>
</a>
</a>
</div>

In this section you can modify image, link and text for banner:

1. In order to change the link for the banner, you have to open the required page and copy its URL. Place it instead of page_url.here in <a href="{{ store url=’page_url.here’}}"> line.

2. The default reference used for the banner image is <img alt="" src="http://osworlds.com/skin/frontend/base/default/’images/media/banner_image.jpg’"/>. Replace the default name of the file with the required one. Make sure new banner image is uploaded, usually it is located under /skin/fontend/default/theme###/images/media/ folder on server.

3. <h>Banner title here</h> is where you can change text for banner. Update it with your own one. Make sure text is placed within <h> <h/> tabs.

Use Save Block button to update implemented changes and open Magento front-end to view modifications.