In this question, the feature of canceling flags is explained. But I don't find a retract flag button after flagging a question or answer. As I'm using the Android app, I'm wondering if this is only implemented in browser.
I'm wondering if this is only implemented in browser.
Correct, you can't retract (the official name for cancel) flags. The reason for this is that it isn't possible via the API, though there is a feature request to enable it. As the Android app uses the Stack Exchange API, your only option is to open the post in a browser and retract it from there.