One of the fastest ways to change a room is to add a new coat of paint. Not only can you introduce an atmosphere-creating color to the room with paint, but you can also just use a fresh coat of a light color as a way to make the room look clean and bright, which is a great basis for any décor.
When re-painting a room, make sure to pull furniture into the center of the room and cover it with drop cloths, and be sure to protect the floor from any drips or spatter. Prep work is key to a good finished product: have clean walls, fill and sand old nail holes and cracks, and be sure to use a primer for the best coverage. Once the paint is dry, have fun putting things back together and redecorating!