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.