Normally, when you sync your bank account with QuickBooks it will download for 90 days transactions. And any transactions prior to that, you will need to check with your online banking and see what you can download to your computer device. You can import into QBO the following files:
Download transactions: CSV, QFX, QBO, or OFX format only.
CSV is in Excel format.
To manually import from your computer device to QuickBooks: In the left navigation dashboard, select Banking > File Upload at the right side (left of Add Account).
Also, read this article for more information:
Import bank transactions from Excel CSV file to QuickBooks Online
In case if it’s not available for bank downloading the above file formats, you’ll have to enter the transactions manually in QuickBooks. You can also use Bank Register (dashboard > accounting > choose bank account from the chart of accounts > view register under action column at the far right).