Currently, the ability to edit transactions that are downloaded from your connected bank account is not available. What you can do is to exclude those check transactions. Then, manually re-enter it with the correct amount.
Here's how to exclude:
Go to the Transactions menu.
Search those incorrect transactions.
Double-click on the transaction to see its details.
Mark Exclude this transaction.
Once excluded, you can now manually add the transaction with the correct amount.
Click the Transactions menu.
Select the Add Transaction button.
Enter the transaction details.
Just a heads-up, manually added transactions are recorded in the Cash accounts.
To learn more about managing your income and expense transactions, you can read through these articles: