My son received a similar letter in January, however it was specifically from USMA not DoDMERB. The letter came with a form to fill out and submit. From the date of notification, he had about 3 weeks to get his BMI up and submit new data. After drinking protein milkshakes daily and having steak for dinner every night for 3 weeks (and after mom and dad each gained 15 lbs just watching this process), he went to a local Army recruiting office to be weighed and he met the requirement.
He did also reach out to our local USMA recruiter/contact for some guidance when he first received the letter. The form could have been filled out by his regular pediatrician, school nurse, Army recruiter, and possibly a few others. You can't fill it out yourself- someone else needed to certify and confirm the new numbers.
The letter you received may be different . . . I hope this is helpful! Best of luck to you!