To my knowledge, the Desktop App will only show updates for products you have activated.
Go to the Education Site and go through the motion of downloading 2018. When it gives you your serial/product key, go to your Autodesk Account and find Maya. Select Downloads > Updates and Add-Ons > Update 2.
https://manage.autodesk.com/You may not see the Maya 2018 in your account unless you activate it on your machine first.
Cheers,
Mike