Cuéntanos qué está pasando:
I can’t use your repository “GitHub - freeCodeCamp/boilerplate-npm: A boilerplate for the freeCodeCamp curriculum.” to implement it in my page and start with the course “Back End Development and APIs”. If I clone the repository, github doesn’t upload the changes because these files already exist in github, if I make a submodule I can’t upload the changes.
Desafío: Gestión de paquetes con NPM - Cómo usar package.json, el núcleo de cualquier proyecto Node.js o paquete npm
There is no need to update/upload the files to GitHub - you are expected to host/serve the app on a public domain. If you are unsure what this means, I suggest following the link pointing to replit.com, as this will automate a lot of the setup for you.
If you really want to upload your code to GitHub, then you need to fork the freeCodeCamp/boilerplate-npm repo to your own account, and change the upstream for the Git repository to point to your GitHub repository.