How to reduce knee swelling from running too much?

See your doctor: Your knee should not normally swell from a long run. See your doctor to assess if you may have an injury within the knee such as a meniscal tear. If it is arthritis, then you need to consider decreasing your running and looking towards low impact activities such as swimming/biking. You may also ice your knee after a long run/workout.

Ice & NSAID's, rest: To relieve and reduce swelling/inflammation you could use knee sleeves, therapy, rest, ice and over-the-counter pain & antiinflammatory meds. More importantly the proper diagnosis and the reason for the swelling needs to be established.