It is pretty easy, I've created myself a two-lane one-way tram track this way.
1. Open the road you want to edit in asset editor (click New, not Load).
2. On the "Road properties" tab, find the Lanes category.
3. There, find the lane that goes 'back' and click it (if you have right-handed traffic, it'll be the one with negative position). If it is the right one, the first thing (Lane direction I think) will say Backward. If it says Forward, the it's on the other side of the road.
4. Change Backward to Forward
Note: don't do steps 3 and 4 for pedestrian lanes
5. If the road has multiple lanes in each direction, repeat this for every Backward lane.
6. If you've done everything correctly, the road will show an arrow pointing in the direction of the road when you build it.
7. Repeat steps 2-7 for Elevated, Bridge, Slope and Tunnel tabs (the first was the Basic tab)
8. Hooray, you have a road!