You can learn MEAN learning individual technologies first. Then you can build some small applications to get some good hands on experience of how to build applications using these technologies. So if you wish to learn MEAN stack you should learn each individual technologies first and then learn how to combine them to build web apps.
You can read this blog that explains what MEAN stack is.
You can also get started with MEAN stack by learning individual technologies, like:
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.
Also you can learn Angular by doing this Angular course.