Debit order date change

I have just been on the call with Nedbank now wanting to change my debit order date. They sent a verification code to my old number, the one that I have asked them before to change as well as the address. This should have been done 6 years ago. Now they say I need to go change my details at the branch as they cannot help me over the phone or do it on the money App. I only have the loan with them, I'm not banking with them. Clearly there has to be a way of changing the debit order date without going to the branch.