What caused it?
There are many things you can do.
But first you may want to know what caused it as there are many causes for swelling and pain in the knee, such as injury-related meniscus tears, ligament tears, blood collection, or infection, arthritis
, crystal-induced (gout
, etc), and other inflammatory causes. Start with ice, anti-inflammatory or pain meds, and leg elevation
while waiting to see a doc.