Changing Subscriptions
To change forum subscriptions, click on “Forum Subscriptions” in the grey “Welcome back” box at the top of the forum.
Underneath the name of the subforum, you will see two options: Unsubscribe and New Thread. You can click Unsubscribe below any subforum listing to remove the subscription.

To change threads subscriptions, click on “Thread Subscriptions” in the grey “Welcome back” box at the top of the forum.
Underneath each thread title, you will see the type of notification selected for that thread.
On the right, there is a checkbox next to each thread. To select one or more threads, click on the checkbox next to each thread you want to change. To select all the threads on the list, click the checkbox above all the threads.
Once you have selected the threads to change, find the With Selected dropdown box on the right below the list. Click it and choose the option you want to apply to all the threads you've selected.
Click "Go" when you have made your selection to apply the changes to all the selected threads.
To quickly remove all subscriptions, click "Remove All Subscriptions" on the left below the list of threads.

You can also find this information in "Control Panel" (UserCP) at the upper left.
Look for Miscellaneous on the left, and find Subscribed Threads or Forum Subscriptions.
When you click Subscribed Threads, you will see a list of threads to which you are subscribed.
When you click Forum Subscriptions, you will see a list of subforums to which you are subscribed. You can click Unsubscribe below any subforum listing to remove the subscription.