If you push changes with arc land or arc commit, this will be done for you automatically. These commits should normally be recognized within a few seconds.
If your repository is hosted on Phabricator, this will also be done for you automatically.
You can schedule an update from the web interface, in Diffusion > (Choose a Repository) > Edit Repository > Update Now.
You can make a call to the Conduit API method diffusion.looksoon. This hints to Phabricator that it should poll a repository as soon as it can. All of the other mechanisms do this under the hood.