Go to your job feed. In the lower right-hand corner, there will be a thing you can click on to load the tracker onto your computer. Remember when you are using the tracker that you need to put what you are working on in the memo field. You can also check your times in the timesheet details in Reports. You can only load manual time if the client allows that.
Please check this page for the system requirements and link to download Upwork Desktop App. Note that time tracking is not available on mobile devices.
I'd like to also use this opportunity to remind that freelancers need to have an actual photo of themselves as their profile picture. Please see this article for more information.