In git, the upstream version changed the folder to submodule. How can I pull it to my version?

Question

For a certain project, the upstream version created a separate library in a subfolder and changed it to the form of submodule

I forked this project and now I need to update it, but I can’t directly git pull https://github.com/xxx/xxx.gitget the submodule code

What should I do?

Also, I have made my own modifications to some codes in this submodule. Do I need to fork another library?

Project address:
Upstream: https://github.com/Fluorohydride/ygopro
Mine: https://github.com/mercury233/ygopro/tree/server