I can't delete a bank account

Here is the place in your account were you can update your deposit method:

https://sellercentral.amazon.co.uk/sw/AccountInfo/DepositMethodView/step/DepositMethodView?ref_=macs_aidepvw_cont_acinfohm

In order to be able to do this, you will firstly need to verify your identity by entering the previous bank account number in full.

If you are unable to provide it for any reason, open a case with the Seller Support and they will offer you a different way how to do this - by supplying some documents to verify your identity.