If you have an automatic transmission, there's no need to shift gears yourself. The car will automatically do it for you under most circumstances. However, if you're driving up a steep hill or towing a heavy load, you may want to switch to a lower gear manually.
To do this, put the car in low mode by moving the shifter into the lowest possible position. This will make the engine work harder and give you more power on steep inclines or when accelerating from a stop with a lot of weight behind you. Just be sure not to overdo it - shifting too often can put strain on the engine and wear down your brakes.