There are two things you'll have to consider before you can change your statement ending balance. Let me share them with you and help you from there.
First off, if you've just started reconciling your credit card and it's still open, you can easily change your statement ending balance. Here's how:
Go to Accounting.
From the Account drop-down menu, select your Credit Card account.
Choose Resume reconciling.
Click Edit info.
Enter your correct statement ending balance in the Ending balance* field.
Once completed, you can now change your statement ending balance when reconciling your credit card account.
Meanwhile, if you're done and recently closed the reconciliation of your credit card account, there isn't to change the ending balance you've entered. Just make sure to enter the correct statement ending balance when you reconcile again your credit card account.
Please let me know if you have any other questions. I'll be always around ready to help.