Hi Trung, I am Paulo. I'm an Independent Advisor and an individual like you inclined to help.
Microsoft announced that Windows 10 is a major upgrade that's why they go directly to 10 and skipped 9.
Here's some news about it:
https://www.businessinsider.com/windows-10-not-...
(Standard disclaimer: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.)
I hope this info helps.