Install magick tool:
sudo apt update
sudo apt install imagemagick
Copy *.png files to a folder, for example:
win16x16.png
win32x32.png
win48x48.png
win64x64.png
and run the following command:
convert *.png winapp.ico
Creating Windows Application Icon on Ubuntu 22.04
Setting the Application Icon
https://doc.qt.io/qt-6/appicon.html
Installing Magick:
https://techpiezo.com/linux/install-imagemagick-in-ubuntu-20-04-lts/