It really is not a big deal in the overall scheme of things.
Amend the account number and the balance, complete it and return it to the bank with a polite note confirming the error . You could suggest that they might check if the statement of means with your account number and outstanding balance has gone to someone else.
I work on the assumption that people can make mistakes - it just happens- and I try not to get too excited about it. Plenty of other more important things to worry about.
mf