The manual says you can change the brightness and the height of the HUD using the multi information screen, but it doesn't say how. So, how?

 

Northside Lexus's Melissa O'Connell's 2023 Lexus ES Full Tutorial - Deep Dive video starting at the 51:20 time mark discusses the Multi Information Display (MID) settings to adjust the HUD brightness and vertical position:

Melissa's deep dive videos are pretty long but if you expand the video description she includes a pretty detailed time mark index so you can quickly jump to a specific topic you want to find the discussion on that topic.