It will eventually get pushed to all Windows 10 users but if you want it early Microsoft's Update Assistant and Media Creation Tool are both available here.
Windows 10 1903 still has problems with removable drives, i.e. USB drives, so you should use the Update Assistant.
Before you start the install take a backup.
Then turn off Controlled Folder Access. I still don't understand why Microsoft doesn't whitelist their own applications.
If you're running Windows 10 Pro, the next thing to do is go to Settings / Windows Update / Advanced options. In the "Choose when updates are installed" section, reset all the days to 0. DON'T SKIP THIS STEP.
The upgrade should run about an hour.
When you're done, turn Controlled Folder Access back on. If you're on Windows 10 Pro, reset the days in "Choose when updates are installed".
So what's new in Windows 10 1903? Read these articles from Lifehacker and Thurrott.
And stay tuned to this blog because there's lots that they didn't cover.
Update: If you don't see 1903 at Microsoft's update page, try this link. Be aware that there might be a reason that it's not offered to you.
One of those reasons could be the following:
Yeah, I upgraded anyway and I've seen the problem.