Hi, I assume your viewing the about us page via the customiser tool. If that's the case have a look for the sections in your sidebar to see if a button or text can be added that way. You may be able to add some HTML text to the page via a section.
Failing this then you will need to customise the code for the template used on the about-us page. You can find the template by looking at the page itself in shopify admin, on the right hand side it will have a page template. Most themes have a custom about us template which I would assume your using. Then you would navigate to Online Store, and choose Edit Code in the drop down list next to the customise button.
Once loaded, search in the top left corner for about, and you'll likely see a snippet or template returned called about-us. Once opened this file, you could add the HTML code.
As a side note, please make sure to duplicate your current theme first as a backup before making any changes to the code.
Im more than happy to take a look or add the code for you if you wish.
HTML CODE - This will be the same for the HTML editor in a section side bar, or if your editing the actual template code.
<a href="*THE LINK HERE*">Contact Us</a>
Hope this helps. Let me know if you need further assistance.
Yes, you'd need code but it's not a difficult one.
I presume you want to show it on the header, correct? f you'd like to send your website's URL I can show you how to do it.