Depending on the degree, most runners often just back off. Either less distance or slow the pace or both. If you cannot walk on it, you should not run on it.

Wearing a brace -- those wrap around velco ones often help. If it does not provide enough support, etc, you may not want to run on it.

Wearing a brace all day is also helpful. The compression helps reduce the swelling and it warms the knee. Resting the joint -- elevate it to reduce swelling.

Swimming might work -- provides exercise to strengthen the joint and is very low impact.

Other than that really not a whole lot you can do. Again if you cannot walk -- weight lifting on the knee is not a good thing.