Refreshing a Data Source is simple for both Live and Extract Data Sources.

For a Live connection, right-click on the Data Source and click on Refresh. That's it.

For an Extract, clicking on Refresh won't work because the data in the extract file (.hyper) hasn't changed. Remember that when you create an Extract, you create a copy of your dataset in a Hyper file. Even if the data changes in the dataset, the data in the Hyper doesn't. To refresh an Extract, you need to refresh the Hyper file. To do so, right-click on the Data Source name, go to Extract, and then Refresh, as in the following screenshot:

When refreshing an Extract, Tableau warns you that the data will change. If you click on Yes to continue, Tableau establishes a new connection to the dataset and imports the data again. At the end of the extraction, your Extract is refreshed.

What if there were some modifications in the dataset between two refreshes?