First, you go to the datasource tab and then add new datasource, then you choose your dataset. Then you go to a random viz and right click one of the datasources and replace your old dataset with the new one. You can then continue to create a new parameter and calculated field and publish it.

A dashboard parameter is just a parameter which is shown on a dashboard, similar to a filter. You can do this by right clicking any viz on your dashboard and hover over parameters, this will show all of the parameters in the workbook, and to show it on the dashboard, click the desired parameter from the list.

Also, parameters are always shown in the whole workbook, no matter which datasource or viz.