Modifying a JSON via Javascript

What’s up everyone.
I’m trying to create a web application that allows you to create an animation based on a template. So let’s say I have a json animation what has a text tag somewhere, for example, the “video” starts greeting the visitor by their name “Hi Daniel”, so I want this name to change according the the logged in user’s name.
I’m trying to find documentation that would allow me to do this. I’m quite lost as to where to start.
For now I’ve been googling but finding stuff which is too high level.
I can code a fullstack web application with React and Node.js in the backend.
Any guidance on this?