How the beginning balance is calculated
The opening balance on your statement is the total amount your bank has recorded for all the transactions in your account at the beginning of the statement period.
The way QuickBooks Online calculates the beginning balance for an account depends on whether the account has ever been reconciled before:
Previously-reconciled accounts: The total amount of all reconciled transactions, regardless of date
New or previously-unreconciled accounts: The account's initial balance
When your QuickBooks Online beginning balance doesn't match the bank statement's, it's called a beginning balance discrepancy.
Tutorial
Select the Gear icon at the top
Under Account, click Reconcile
Copy the Beginning balance number
Top right click on the Search icon
Click Advanced Search
Include the following criteria: All Transactions Lines, Line Amount, Equals, paste the original beginning balance number
Click Search
Click on the Type (ex: deposit) that has that same amount
Edit to the correct Amount
Click Save and close
Notification pop up: click Yes
Here's a video tutorial on how to create an opening balance from scratch.
Here's a Youtube Channel of more recently updated video tutorials for Quickbooks Online.