Help - can't upgrade my yahoo mail


New Email
I have several yahoo accounts, some important for work. They are all Yahoo Classic mail - which is about to be discontinued in favour of a new version. When I click the upgrade button I go to the new terms and conditions screen but the button for clicking yes to agree is just blank and does not operate. This is the same no matter which account I try and upgrade. I am desperate because I am about to lose access to all these accounts and Yahoo does not respond. What's the problem ?