I want exit from this group

1. If you are bothered by the amount of email messages from forum activity here, then you can stop that from your Profile Preferences. Just un-check the boxes for the types of messages that you no longer want to receive, then Submit (Save). Especially “Following” can generate a lot of emails.

2. If you “subscribed” to a topic [sub-forum] (e.g. iPhone), and do not want to be subscribed to this topic anymore, then you can click the “Following” box near the top of the topic page, to unsubscribe. It would then read “Follow” as an unactivated option.

3. For threads [discussions] in which you participated, or otherwise wanted to be notified of, then the bell icon near the top of the thread page would be solid. Click that bell icon (again) to unsubscribe. It would then show as open.

You do appear to be subscribed to the iPhone forum. That's going to generate a lot of notifications. Step 2 in Urquhart1244's excellent answer tells you how to undo that.