All cordless impact drivers have a reverse function which allows the chuck to turn backwards as well as forwards.
On most models, you can alternate between forward and reverse by pushing the forward/reverse button on the side of the tool. This button is usually located on both sides of the tool (so it can be pushed with either your index finger or thumb) and just above the speed control trigger.
The direction in which you push the button to select reverse may vary depending on the make and model of your tool. On some models of cordless impact driver, pushing the forward/reverse button into the central position locks the tool ‘off’ preventing the chuck from turning.