A bank can never change an incoming Direct Debit. When you sign a direct debit, you ask the bank to pay unspecified amounts from your account at the request of the payee (in this case, the building society). Your bank has only two options - to pay the amount requested by the payee, or reject it. They do not have an option to partly pay it. You must deal with the building society if you want to change the amount.
You can cancel the direct debit, and pay the building society a lesser amount by other means, but that may have other consequences.