How to edit archive page’s content inside wordpress dashboard?

Making it a taxonomy and allowing the client to edit the term title & description would be an option I’d consider.

The theme may or may not be using a custom template-file to control the cpt-archive’s front-end.

Using a page-template would be ill-advised because by loading the post-type and the page at the same url, problems can arise.

Using theme-options would be a good way. Having a theme option in combination with a custom archive-template for the post-type archive could do it.

For how to create theme-options, See this example: https://codex.wordpress.org/Creating_Options_Pages#Example_.232

& for custom-page templates, this has you covered: https://developer.wordpress.org/themes/basics/template-hierarchy/#visual-overview