You can easily change gear D and B while driving. Drive D is used for normal driving, while drive B is mostly used when driving down a long hill and to increase regenerative braking.
B mode ramps up the regenerative braking, which can improve fuel economy and save wear on the brakes. We'd recommend using this when you're driving in heavy traffic around town or travelling down a steep hill. At higher speeds, it's better to stick with D mode and let the car coast when you lift off the accelerator. You don't want it slowing down of its own accord at motorway speeds. You'll soon learn the best times to use each different mode but, if not, stick with whichever feels the most natural to you. Neither will cause damage.
Please notice that when you change D to B and vice versa, come to a complete stop and keep the brake pedal depressed. Operating the shift position before the vehicle has come to a complete standstill can damage the transmission.