//Some code before...

//Load some txt files in the spinner
list = app.ListFolder( "/sdcard/Folder", ".txt" );
spin.SetList( list );

// Returns the currently selected index from the spinner
function getSpinItemIndex()
{
    return list.indexOf(spin.GetText());
}

// select the next file in the spinner
function select_next_spinner_item()
{
    var listIndex = getSpinItemIndex();
   
    // Make sure there is a next file
    if(listIndex+1 < list.length)
    {
        var file = list[listIndex+1];
       
        // Show the next file in the spinner
        spin.SelectItem(file);
    }
}