As usual, you can either use the built-in updater from the Utilities menu, or you can download and install directly from the website: http://www.getpaint.net/. There’s no need to worry about removing the old version; that is all handled automatically.
Here’s the changes from v3.5.8:
- Improved: The "Auto-detect" bit-depth setting for PNG, BMP, and TGA now also determines which bit-depth to use based on which one produces the smallest file size, as well as which ones can save the image without losing fidelity.
- Improved: You can now use Ctrl+0 as a shortcut key for View -> Actual Size, in addition to Ctrl+Shift+A and Ctrl+Alt+0.
- Fixed: Some text in the DirectDraw Surface (DDS) Save Configuration UI was not being loaded.
- Fixed: Some DirectDraw Surface (DDS) files authored with other software (e.g. Unreal 2004) could not be loaded.
- Fixed: In some rare circumstances, clicking on the Save button in the toolbar would crash.
- Fixed: The Korean translation has been added back in, with the help of Bing machine translation to cover the few remaining strings that were untranslated.
Also, fixes since the 3.5.9 Beta include a few compatibility issues with plugins.
Enjoy!
paint.net blog 
Thanks for your work. I now can’t use a machine without installing Paint.NET !
And as a .Net developer, PDN taught me a lot (your application is one of the biggest and useful Winform application I know). And thanks again for your explanations a few years ago when you decided to not release the source code anymore, I totally understand your POV. And as a FOSS developer, I’m definitely thinking of doing the same for my latest application.
Please, don’t remove the Ctrl+Shift+A in future releases. It’s a lot easier than Ctrl+0: many people use graphics programs with the right hand in the mouse, and the left hand at the left of the keyboard. Ctrl+Shift+A can be pressed with three fingers of the left hand, while Ctrl+0 makes you either move the left hand to the right of the keyboard, or lift the right hand from the mouse. In fact, when I design the keyboard accelerators for my programs, I try to assign the most used ones to keys on the keyboard’s left half (Ctrl+Z,X,C and V follows this pattern!).