You can open the following link to download the application: https://quickbooks.intuit.com/apps/. Scroll down to the Get the QuickBooks Mac or Windows App section, and then click the Free Download button.
Once downloaded, you'll see a shortcut icon for QuickBooks Online on your desktop screen. Simply click it, log in using your credentials, and you're good to go. I'll be sharing with you this article: QuickBooks App for Windows and Mac: General support. It contains more details regarding the application's system requirements.
If I can be of additional assistance while working with QuickBooks, don't hesitate to let me know. I'd be more than willing to lend you a hand. Thanks for dropping by.