What is the best way to manage fluid on the knee?

Depends : On if the fluid is acute and the cause of the accumulation .If the need is to establish etiology, a tap of the joint for analysis may be indicated. If it is for treatment a tap may also be necessary and sometimes with injection to the knee. Management of the cause of the knee effusion is also necessary

See a doctor: It depends what the fluid is really and the only way to diagnose that is by seeing a doctor. For instance if the fluid is blood from a trauma or from being on blood thinners then this is managed differently from if it is an arthritic process.

Swelling in knee: Fluid on knee could be caused by infection, injury, gout, arthritis, bleeding in joint or a tumour. Fluid has developed suddenly or a person is unable to weight bear or they have high fever or person is feeling too unwell they should see doctor urgently .

Knee effusion: knee effusion result from traumata or other causes as inflammation or osteoarthritis (wear and tear) usually history, physical examination, lab test and imaging establish the diagnosis, some times tapping the fluid can help a common cause of knee effusion on wear and tear in which case weight loss, staying active can be useful, you need to see your doctor to get the right advise for your condition

Drain it: The best thing is to see your doctor, have it drained and checked for infection

Swollen knee: You are asking about a joint effusion. There are many causes. Most concerning is infection. In the setting of trauma it could be blood or fat from a fracture or edema. I could be crystals form a metabolic issue. The only way to know fore sure is to have it drained and examine the fluid. Management will depend on the etiology.

Depends on cause: Injury? Gradual onset? Painful? Most common causes of gradual onset, mild - moderately painful swollen knees in someone your age is either osteoarthritis or an injury. Avoid further injury for a while, paracetamol and a walking stick may help. Longer term losing weight if you carry excess can help. It would be good to have an online consultation to clarify your situation

Commonly = overuse: Most common reason for fluid on the knee is over-use which causes a collection of inflammatory fluid to collect. In this case the treatment is to rest it, stop the movement/exercise which is irritating it, elevate it whenever possible. Frequently, with sufficient rest it will settle on its own. This may take several weeks however.