Almost all 2D engines are the upper left corner is the origin，Is there any simple way to change the Y -axis downward.
Not really. I mean you could try hacks like scaling everything by (1, -1, 1) and shifting, and then scaling all units again (1, -1, 1)… but it will definitely cause more confusion and work, due to being inconsistent with most of CGE behavior.
I realize this may be non-standard, but we deliberately chose convention “Y goes up” for everything – UI, 2D viewport, 3D viewport ( Which way is up? | Manual | Castle Game Engine ).
This paid off – in the sense that we have everything consistent, in both 3D and 2D Y goes up, and they have right-handed coordinate system (like in glTF, X3D, OpenGL…), and it allows to make mixed 2D/3D games (where player sees mostly 2D but you occasionally utilize the fact that actually everything in 3D) without weird issues.
So, for this – my advise would be to just embrace and adjust to this convention. Then all CGE API will work consistently.