Have to admit I don't know if MyPay looks different for Cadet pay but on the retiree page when you sign in you get the block to look at your statement, and also in that block is Pay Changes bolded (allotments, direct deposit, address) also bolded is taxes (federal, state).
Click on direct deposit and it shows the current bank with routing number and account number. Below that is a "change" box -- click on that and you can enter a new routing number and account number.
Hope that works in cadet system