First, download transactions from your bank by logging in to your bank's website. Then, follow the given instructions on how to download transactions and save the file on your desktop or anywhere you can easily find it. Once done, upload them to your book by following these steps:
Sign to your QBO account and select Transactions on the left panel.
Click Link account drop-down arrow.
Select Upload from file. Then, click the file you've saved on your computer.
You can use these articles as additional reference in fixing bank upload errors, categorizing transactions after imported them, and the reconciliation workflow in QBO: