By default the ability to edit Symbols is not active (a somewhat recent change from Webflow) as it could cause collaborators to change something in one place and unknowingly edit it in many other places.
Thankfully you have full control over which elements can or cannot be changed within the Editor, so just make sure that anything contained within that symbol has the “Collaborators can edit this element” setting checked in the Element Settings panel:
Once you update that setting, the elements inside will react just like any other element that can be changed—with updates made across all instances of that Symbol
Edit: As a side note, this functionality of including a page with Symbol elements for clients to edit is super useful for all types of situations. Say you have a couple different CTA blocks that appear at the end of specific blog posts based on a toggle in the collection. Instead of needing to figure out which of your blog posts contains a specific CTA, you could have the Symbols included under a “CTA Block” section of the client Symbol page and allow them to update all instances in one place.