Tip: You can also directly upload to PowerBI service, see how here.
Start by refreshing your data (see here if you need a new token) then select Publish.
If you haven't already, then you'll be asked to save your report and sign in. (This is the same sign in you use for PowerBi Service)
Once you've done that you'll need to pick a 'Workspace'. Pro users can create custom workspaces in PowerBI Service.
Select the workspace you want to use and it'll publish your report. If the report already exists then it'll simply update/refresh your data.
Assuming the token is still valid you can refresh the dataset from PowerBI Service. Once the token expires you'll need to update it in PowerBI desktop and Publish to service. You can read on how to get and update your token here.
Tip: you can read the full documentation from Microsoft on PowerBi Service here.