To unallocate

1. Go to Suppliers, select the required supplier, then select Activity.
2. Select the payment you want to unallocate, then press Unallocate or Unallocate transaction.
3. Press Yes.

The option is unavailable

1. Check that you have access to the 'Unallocate payment' option in Settings, User Management.

If you have access, check why you can't unallocate a transaction.