The example was designed to show off 4K support on the board so there isn’t a single place to change the output resolution. The easy way to adjust the output resolution is in code cell 4 of
mixer.ipynb. The vertical and horizontal resolution of the output is set there 3 times in total (for the HDMI output, the video mixer and the test pattern generator). Updating those three to 1920x1080 will give you a 1080p output.
The rest of the demo is built around tiling 1080p layers though so now the output is 1080p you will only be able to show one window at once. The VideoMixer isn’t able to scale down layers so you would need to update all of the layers to operator on 960x540 images.
The other option would be to modify the hardware design and add a video scaler after the video Mixer and before the HDMI frontend that scales the 4k output of the video mixer down to 1080p. That way you would have the same demo but running at half the output resolution.