Making an external running fullscreen program go to windowed using Qt or C++ commands?

[pditty8811]

Making an external fullscreen program go to windowed using Qt or C++ commands?

I'd also like to know the vice versa, from windowed to fullscreen.

Is this possible with Qt or C++ commands executed from my Qt app to make an external program from fullscreen to windowed.

Again, I'd like my Qt app to have the command but the command to effect an external program that is running alongside my Qt app.

Thanks.