If you are using Windows 10 Insider Build 14986 and above you will find that the Classic Paint is no more and 3D Paint Preview has replaced it. Many of us are accustomed with Classic Paint so the latest app is making problem in using for them.
So here is the resolution:
1st Method: Press Win key and find Paint 3D Preview > Right click on it and select Uninstall.
My experience: There is no Uninstall option in 14986.1001-so this method is useless for me.
2nd Method Registry hack:
In the Start Search type in Regedit and press Enter Key
Locate to the following registry location:
My experience: I didn’t find Settings under Paint then right click on Paint and hover on New and thereafter select Key in submenu and made a new key called Settings. Right click on Settings, hover over New and choose DWORD (32-bit) Value on the Submenu and named it DisableModernPaintBootstrap. Here I changed the value from 0 to 1 by double clicking on it.
Simplest and safest. Just download and install CCleaner. Click Tools, select Paint 3D-Preview and click Uninstall tab.
If you like to get it back:
Open Windows Store, search for Paint 3D-Preview , click Get tab.