you want refresh the extract or the dashboard?
Paulo Dantas its Dashboard...
You can schedule a refresh in Tableau Server by setting up when to extract, if not you can make it as LIVE connection which will get the real time data.
You can refer below discussion on similar queries:
You will want to use the following tabcmd in order to refresh your workbook.
tabcmd login -s http://localhost -u jsmith -p password
tabcmd refreshextracts --workbook "My Workbook"
You may need to add the --no-certcheck option to your scripts as well.
Here is some more information about the tabcmd scripts