Assuming you have already logged into phpBB. Click on the Forums link found on the main menu.

You will land in a Forum Administration page, where you can add, edit or delete forums and forums categories.

Click on Your first category link as found above.
To manage the forum click on the green icon found on the far right of your screen as seen above.

You can change the name, description of the forum, and click the submit button to save changes.
That’s it, you can see the changes made to the forum.
To delete the forum click the red icon found on the far right of your screen as seen above and click submit button.

That’s it, you now know how to manage a forum in phpBB