While I was entering some bills and marking them as paid, I accidentally marked them as paid by check instead of paid by credit card. Is there a way to change them or do I just have to void them and re enter them again.
The answer to your concern depends on how you want this transaction to be shown in your reports. Voiding changes the amount of the transaction to zero but keeps a record of the transaction in QuickBooks. While deleting it removes the transaction entirely in QuickBooks, therefore, it won't show on your report.
If you want to delete or void them, you can follow the steps provided above by clicking the its link.