Short answer: You can't from within admin.
Yaml files are not editable in Admin.
Admin is an interface to set predefined values in the page-creating yaml (frontmatter in .md, config.yaml, etc).
Somewhat confusingly at first, configuring Admin to set the predefined values is ALSO done in yaml files.
To keep the two separate (page creation yaml vs Admin configuring yaml), the yaml for admin is referenced as "blueprints" (either in the file name, or in the folder containing it).
So to set it up for your clients, you would want to create your own blueprint yaml files to "extend" Admin so that it includes what you want to be made available to clients. That will have to be done in an editor.
Keep in mind - I'm a learner too. I started to pick Grav up ages ago but had to drop it for work, and am just now re/learning again from scratch. Also, I am not a developer. So I could well be wrong in my thought processes. Coming from a hobby SSG background before, it took me a while before I started to "grok" Grav but it is great for quickly building performant websites with admin login - once you have your theming etc setup.
Hope that helps.