Hi emilyfizz40-gmai,
When you connect your bank account in QuickBooks Online via standard feed, it will automatically download the last 90 days of transactions. You can import historical bank transactions manually via CSV file.
First thing you'll need to do is to download your bank transaction data from your bank's website. Once done, make sure that the CSV file includes the following columns only: Date, Description and Amount or Date, Description, Credit, Debit. Doing so, it'll be easier for you to map the fields and import it in QuickBooks.
For more detailed information, you can check on this article: Import Bank transactions via CSV.
Let me know if you have any other questions, emilyfizz40-gmai.