Instagram has added support for Dark Mode in the latest update for iPhone. However, there’s no toggle switch to enable or disable dark mode in the Instagram app. The app follows the settings of your iPhone only, there’s no escaping to that.
If you have set your iPhone to automatically turn on Dark Mode after sunset (or a custom time), then the Instagram app will also follow the same pursuit. It’ll switch to Dark Mode when your iPhone is running in Dark Mode.
So, the only way to turn off Dark Mode in Instagram