Hmmm..it depends on how that image was added...from a cursory look at your website it looks as if that image is within a table. If that image/table is within a content block all you have to do is open up the block in the editor, and click on the "link" icon on the TinyMCE editor to change the link.

It may be that you have a "stack" defined to create the header throughout your site. If that is so, then you can use the Dashboard to go to the stacks, and edit it there.

If for some reason you hard-coded the heasder, then that would depend on where you placed the header.php file. There is one under the /concrete/themes directory but in custom themes that header.php file can be in the /packages/[theme name here]/themes directory.