I don't see a report button anywhere in the Android app. Somebody please correct me if I'm wrong.
> Reporting a channel
> Open Telegram > Open channel > Select channel name (top) > Report > Select the report reason
> If you need to report content by email
> Address it to abuse@telegram.org with a subject like “Report user @username”
In desktop app you can simply click right button on the message or click three-dots menu in the top bar and there will be an option for report.