the easy way is to ask the current superadmin to add you to the superadmin group :
!putgroup <someone> superadmin
then remove the first player from the superadmin group :
!ungroup <1st player> superadmin
An alternative way is to alter the data straight in your mysql database : open the table called "client" and find the row matching the player you want to remove from the superadmin group. change the value of the 'group_bits' column with '0'.
Now as there is no more superadmin, you can type !iamgod again ingame.