When you first connect your online bank and credit card accounts, QuickBooks Online downloads past transactions up to the last 90 days (3 months) worth of transactions for Standard feeds connection. If you connected your bank through Direct feeds connection no historical transactions will be sent to QuickBooks.
If you want to add transactions older than 90 days, you'll need to upload them manually using a CSV file.
Here's how to:
Click on the Transactions tab, and select Bank transaction.
Click on the drop down arrow next to Link account, and click on Upload from file.
Select the CSV file you've downloaded from your bank.
Click on Continue.
In the QuickBooks account ▼ dropdown, select the account you want to upload the transactions into. Then select Continue.
Follow the onscreen steps to match the columns on the file with the correct fields in QuickBooks. Then select Continue.
Select the transactions you would like to import. Then select Continue.
Select Yes.
When you've accepted your transactions, select Done.
Your transactions will now show in the For Review tab and ready for you to be categorised.