How do I change a frame mode from within prototype

[Haroll]

Hi @Suzanne_Wensveen

You are not missing anything. It is just not possible at the time.
But you can upvote this, so it will come to Figma’s attention

Dynamically change modes through prototyping interactions

Currently it’s not possible to dynamically set or assign mode variables to a frame or a document. This would be incredibly helpful when it comes to user settings, like change a theme or language, for example. When prototyping, dynamically setting a mode variable like systemTheme or systemLanguage could then be done through user interactions like clicking a button, to simulate many real-life use cases.