QuickBooks Online (QBO) will automatically download your most recent transactions once your bank account is connected. If you want to forcibly stop the update, I recommend disconnecting your online banking.
Please take note that disconnecting a bank account will delete all uncategorized transactions. If these transactions are older than 90 days, there will be no option to download them again into QuickBooks Online.
Here’s how you can disconnect a bank account:
Go to the Banking menu, and select the Banking tab.
Go to the bank account tile, and click the pencil icon to edit.
Select the Edit account info.
Tick the Disconnect this account on save, and select Save and close.
Additionally, here are the articles that will help you with categorizing your transactions. Once done, reconcile them to ensure your books are balanced: