You may change the look of your office using the Decor menu, which can be accessed in the office itself. It is the first button in the menu on the bottom of the screen. When this button is clicked, a different set of buttons appears at the bottom of the screen, each of which denote different categories of furniture.
Furniture you already own has a stamp next to it. Red labels indicate furniture which is in use. Yellow labels (期間限定) indicate seasonal furniture.
You may buy furniture using the proper amount of coins (given through events and completing research missions) and decor licenses (given through events).