Hello,
content-manager is already installed in your strapi instance. There is no need to install it manually.
It is located under ./node_modules/strapi-plugin-content-manager
To overwrite it’s files, you should manually create a folder under ./plugins/content-manager/*
In that folder you should copy a file from ./node_modules/strapi-plugin-content-manager, now you can modify the file and it will overwrite the original one.