If you have a Reseller account which you wish to upgrade to an Admin, use the following guide.
Replace username with the User you wish to upgrade.
cd /usr/local/directadmin/data/users/username
perl -pi -e 's/usertype=reseller/usertype=admin/' user.conf
cd /usr/local/directadmin/data/admin
perl -pi -e 's/^username\n//' reseller.list
echo username >> admin.list
