The functionality is available starting from the Standard plan.
"Update On Demand" can be executed via the URL View "Bulk Actions" menu (top left). Check the URLs you want to run an update for, and click "Update On Demand". All of the terms under these URLs will be updated.
To update selected terms of one URL in the URL VIew, you can select the terms and click the update icon below.
It can also be done for specific terms of a URL in the URL's Overview & Daily page. (Access from the URL View: URL Box top right -> Pie Chart icon.)
Check the terms you'd like to run an update for and select "Update On Demand" at the "Bulk Actions" drop-down below.
To update the entire URL's terms click the Update icon at the top-right corner of the page.