You can edit the text of the 'Start a Conversation' link that appears at the top of every page in the Help Center by modifying the 'Header' template.
While editing the 'Header' template, search for the following string:
{{ phrase('header.menu.startconversation') }}
Replace the entire string with whatever text you'd like to use for the link.
For more details on template customization, please refer the user guide article on customizing the appearance of your Help Center.
Gurpreet Singh