Edit the "Program" object. ( Either by double clicking the "Program" object, or by right-clicking on it and choosing "Edit" )
You can then choose a machine from the Machines drop-down ( click on the down arrow to the right of the machine ).
You can also choose whether you g-code file will go in the same folder as your data file ( your .heeks file ) or in a specific folder.
To change the file suffix ( .tap, .ngc, .gcode etc see "Configuring post processors" )
You can also choose the output units, mm or inches.
Then click "OK" to confirm your changes