You can contact John, the site admin, if you want to change your username. However, as you are new, you may prefer simply to register with a new name.
Also, if you're a DPhiE, I'll hazard a guess that you're female.
In which case, your alums are alumn
ae, not alumn
i.
Alumnus = male singular
Alumna = female singular
Alumni = male plural
Alumnae = female plural
Alumni is also used for a mixed gender group. You could have 185623 women and 1 man in the group, and they would be "alumni" because of the one man.
<---- studied Latin for wayyyyy too many years