Instructions
Open the operators list text file in the folder containing your Minecraft server's files. It's marked "ops.txt" (without quotes).
Locate the Minecraft username of the operator you wish to promote to administrator.
Cut the name from the list by highlighting it and pressing "Ctrl-X."
Save and close the operators list, and then open the administrator list. It's marked as "admin.txt" without quotes.
Paste the name into the admin list by starting a new line beneath your own Minecraft user name or the username at the bottom of the list and pressing "Ctrl-V."
Save and close the administrator list. When your new administrator signs into your Minecraft server, she will have full administrator privileges.