The front fan bracket is installed with two screws in the top, and is removed by releasing these:
If you want to install 120 mm fans instead of the pre-installed 140 mm fans, you simply remove the 140 mm fans, and rotate the fan bracket 180 degrees for 120 mm support, as the same bracket supports both:
This design allows the 140 mm fans to not be disrupted by the 120 mm fan rails, removing any noise this might cause.