Two ways of changing speed, during your convert or later in the editor. Convert changing: Go to the symbol you want to increase speed for and go to its' properties. There you will find a menu for changing texture, speed, trees, colour etc etc.
Editor changing: Here you will have to mark all the points of the area you want to increase. After that you go to "Triangle Edit" (Default is Point Edit i think), then go to "Speed Settings" and drag the marker to an higher value. ;O