Can I change the content in an Experience fragment or even in a Content fragment once I add it in a page?
Adobe Experience Manager Help | Using AEM Experience Fragments
For example, in above article is it possible to change the title above an image of an Experience fragment once I add that in a page?
Answer to your question is yes to some extent.
1. If we make changes to the "Structure/Content policy" that would get reflected on the page using this template.
2. But,if any changes are made to "Initial content" then changes would not get reflected to already created page whereas if new page would be created, then that new page will have this new changes.
3. Any we make changes to "Layout" depending on the component that are part of Structure or Initial content that would get reflected on the pages accordingly.
Reference Link:- Experience fragment, is content editable after adding in page?
Here is a more step by step fragment article - you can go thought it and try chaning the fragment once its in a page.
Adobe Experience Manager Help | Working with Adobe Experience Manager Content Fragments