It installed straight off for me. Try selecting custom install rather than express.
I opened my Paint.NET program today, after not using it for some time, and got a notification there was an update. I chose to Install on Exit, and let it download in the background. I then closed the program, and it told me it would install the update now; "Optimizing, and installed successfully". Opened the program back up, and it appears to have worked properly.
Glad you found the solution for your problem.
That's how I've successfully updated in the past.
I won't update "now" from now on.