Hi,
I am using KDE Neon 5.12. I want to install plasma 5.13 because there are very useful new features in it.
So, how can i install plasma 5.13 KDE neon? Is it be available in KDE Neon User Edition?
Thanks, for your answer.

It will be available as a normal update once it is all packaged up and pushed to the archive.

I suppose, neon repositories has deleted. I didn't see "https://archive.neon.kde.org/user/".
Now, I added this address in sources.list. But I think, there is a problem sources.list file. I want to change that sources.list file. How can i find original sources.list file?

Check the /etc/apt/sources.list.d directory, the neon repo list file will be in there. you don't need to add anything to the stock sources.list file.