How do I convert or upgrade roads?

In the case of upgrading the size of the road, you can use the "!" button next to your road shape tools (straight, curved, etc).

Unfortunately, this only works on size, at least for now. Due to the fact that you can only decide the direction of a one way road by dragging it, you still have to delete and rebuild a road you want to change to one way. You can also edit the direction of one way roads with this tool, by right-clicking on the one you want to invert.

Note also that this WILL bulldoze buildings that were in the way of the size expansion, and that you'll have to manually move plopped buildings placed along the side of the road you're upgrading or you'll get a "space already occupied" error.

Edit: in this ever changing world in which we're living, CO has added official support for one to two way road conversions and reversing the direction of a one way road. From the 1.0.6b patch notes: road upgrade tool now works between two-way and one-way roads and right mouse-button changes direction of one-way roads

From: http://steamcommunity.com/app/255710/discussions/0/611701360821801977/

Yup, you can upgrade roads. Just open up the roads menu, and click the "Upgrade Roads" button. It's the one that looks like an exclamation point, and I've circled it in the below image. This will allow you to build over existing roads so they can handle a higher volume of traffic.

(image source: http://www.skylineswiki.com/File:Road_construction_1.jpg)

There should be a tool in the road building menu thing that is the upgrade tool.

Select the upgrade tool and then select the road type you want to upgrade to.

Then, simply click on the road section that you wish to upgrade, and voila, it should upgrade.

Just as a side note, if you have any buildings built alongside a narrow road and upgrade to a road that is wider, those buildings will be destroyed.