There is currently now way to change the cache settings per site in Tableau Online, to refresh the cache you need to do two of the following :
- Open the view, and then click Refresh Data.
- Append ?:refresh=y to URL of the view. For example: http://servername/views/workbookname/dasbhoard1?:refresh=y
You could also configure your browser to delete cache at the end of each session, or you could provide your users with embedded views that have the refresh parameter in it (see link below)