Hi Linda to edit CMS content, you can edit it in the CMS, or you can switch the designer to Editor mode ( top bar, right side ), and edit elements that your CMS content is bound to. In that mode, the designer will automatically navigate you to the CMS to do make the changes.
If you’re using the Editor, as in yoursite.com/?edit, everything is WYSIWYG editable.
Styling rich text is a completely different question. You can style any subelement of a rich text element, but not while it’s bound to the CMS. Unbind it first.