Hi Dale - it really depends on how you imported the transactions, ie via import of bank entries, invoices/bills or journal entries.
In most cases you can change or even void items individually, ie just click on them and edit or void. Sometimes only limited editing is allowed and so you have to void them, or, in the case of payments set against a bill or invoice, you have to delete the payment in order to fully edit the invoice/bill.
If you delete an bank entry it is put back into the import to be explained again, so you'll see the bank statement back on the bank page with an entry to explain.
If you have to void (delete) individual bills & invoices, just create them again in the purchases or sales menus.
With regard to opening balances, if you used the "Getting started" feature then you can go back and change most of the entries you made, however whilst this feature allows you to enter unpaid invoices & bills, if you want to edit them you may get a warning that there's no accounting period or VAT scheme in operation, so you have to set them up in Settings.