It should be possible through the Member Centre
Account Details then Edit My Details
Similarly you can create a new direct debit mandate and once that is working the old one will be cancelled
Just select Payment Details and then Update Details
Note that the one thing which can't be changed is the Username although you can and should change the contact email address
This is from the relevant page after clicking on Update
What happens after you complete setup?
The details of your Direct Debit Instruction will be sent to your email address within 3 working days or no later than 10 working days before the first collection.
Change My Direct Debit Instruction
You can change the details of your Direct Debit Instruction by entering your new details in the above form and clicking submit. When your new details have been confirmed your old Direct Debit Instruction will be automatically cancelled.