The fact that the name of this forum contains the word "training" doesn't mean that it answers all kind of questions
from teachers. Rather, it is devoted to question about the training courses offered by Microsoft. Unless you want a recommendation for a MOC course that teaches Windows, it's better to ask your question in one of the
Microsoft Answers forums.
That said, if I had the same problem as you I would save a copy of any documents that I wanted to keep, and then format my system disk and perform a complete reinstall of the desired version of Windows from the original install media. I understand that
this probably doesn't match your criteria for "easily", but at least it is "reliable" which is often much more important.