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Once your bank is integrated with QuickBooks Online, then you can connect your bank and download transactions automatically. However, if your bank is not integrated, need not to worry as you can download a .CSV file and upload it into your account.
To connect bank account, here's how:
Go to Banking tab in the left navigation panel.
Click either Connect account or Add account button.
Enter your login details that you use for online banking, then click Continue.
Click Agree.
Choose Connect.
To import your .CSV file, here's what you'll need to do:
Go to Banking tab in the left navigation panel.
Click File Upload button.
On the File Upload page, click Browse and choose the appropriate .CSV file.
Click Next.
From the QuickBooks Account drop-down, choose the appropriate bank account you want to import your transactions to and click Next.
Map the statement fields according to your .CSV file's format.
On the next page, all transactions in your .CSV file will be displayed. If you'd like to exclude any transactions, untick the checkbox next to those transactions, then click Next.
Choose Yes on the notification asking if you want to import now.