Assuming you have already logged into phpBB. Click on the Manage user’s link found on the left side of your screen under the quick access.

You will find a screen like this below, enter a user name here, or use the find a member tool to locate the username and click submit button.

You will find an overview page where you can edit the user’s profile information, select the options to edit

In this case let us choose the profile, so select profile, and make the necessary changes in the user’s profile and click on the submit button.
That’s it, you now know how to edit a users profile in phpBB