There are lots of ways to do this. First make sure in the data point settings you have made the modbus point settable. Next on the graphic views add the point with one of the widgets. You can use a simple point to start with. Check both the Settable Override option and the display controls option. Now when you view the graphic view and hover the mouse over the item you will see a little wrench popup to allow you to set the points value.
You can also use a server side script to do all sorts of things. Here are some example scripts that would be helpful: http://infiniteautomation.com/wiki/doku.php?id=graphics:mango_graphic_views:server_side_script_examples