Bank Transfers do not have the option for Period Correction.
The following two options would be available:
Unposting the Bank Transfer, selecting the new period and re-posting

OR if it is already reconciled on a Closed Bank Reconciliation
Post a Bank transfer for the same amount and Period but the opposite way around, effectively reversing the initial Bank Transfer.
Then post a new Bank Transfer as per the original one, but now with the correct Period.