Using the same mobile phone number or email address you used when creating your account, log in at https://client.paypaplane.com
Click the Agreement you would like to update with your new Payment Method.
You will see the details of your Agreement listed, find the section "Payment Method" and click the little pencil to the right of that heading.
Select your preferred method, enter your details (ensure they are correct, as incorrect details will result in failed payments and possibly incur extra fees) and agree to the DDA (Direct Debit Agreement).
Once agreed, your Payment Method change is complete, and all future payments on this specific Agreement will use this new Payment Method.