Swelling in the knee can be caused by a variety of factors, including injury, overuse, or an underlying medical condition. Here are some steps you can take to reduce swelling in the knee:

1. Rest: Avoid activities that may have caused the swelling and give your knee time to heal. Resting the knee can help reduce inflammation and pain.

2. Ice: Apply a cold compress or ice pack wrapped in a thin towel to your knee for 15-20 minutes at a time, several times a day. This can help reduce swelling by constricting blood vessels near the joint and decreasing blood flow and inflammation.

3. Compression: Wrap an elastic bandage or sleeve around your knee snugly to prevent fluid from getting worse. Be careful not to wrap it too tightly, which may cause swelling in the lower leg and foot.

4. Elevation: Prop your knee and leg up above the level of your heart as often as you can. This helps to reduce inflammation and drain excess fluid from the joint.

5. Take anti-inflammatory medication: Over-the-counter pain medications like NSAIDs or acetaminophen can relieve pain and reduce inflammation. Talk to a healthcare provider before taking pain relievers for more than 10 days in a row.

6. Do gentle exercises: Isometric exercises contract a particular muscle or group of muscles without causing the joint to move. A physiotherapist may develop a plan of these exercises to help strengthen and support the muscles around the knee while reducing fluid buildup in the joint.

If the swelling persists for more than three days, worsens, or if you experience severe pain alongside the swelling, seek the advice of a medical professional. They may recommend additional treatments such as physical therapy, aspiration, or surgery depending on the underlying cause of the swelling.