I am experiencing ankle swelling. apply heat or cold?

Depends: If this is from an acute injury like a sprain, cold would be better. If this is more chronic or happening at the end of the day you would be better off using a compression sock. Let me know if I can be any more help.

Either way or both.: Depending on the proximal cause of the swelling, the ankle can be treated with either heat, bold or both. Generally cold is for a more acute or recent injury. Heat can help with a chronic issue, mainly to increase blood flow and to promote healing

Cold: At 26 y/o you should not be having swelling unless you are very overweight or wearing non-supportive shoes. If neither is the case, I would speak with your primary care doctor.