Launch
dde-control-center. The window should have the sandwich symbol (≡) somewhere (allegedly in the bottom right corner). Click on it and follow your common sense. I don't use Deepin, so I don't know the exact naming and layout, but it's probably there.
A big problem with Deepin is that large chunks of its documentation appear to be poorly translated from Chinese to English, so at times difficult to understand.
Another thought. The window manager in Deepin is dde-kwin. From its name it looks like a fork of the KDE window manager, so maybe
this also applies here. You can open
Display Settings and
Corner Settings by right-clicking on Desktop and selecting them from the menu.