Hi,
The information provided is not sufficient.
If there is swelling in one leg then it is a local problem, or if it is in bith the legs then it is usually a systematic problem so in such cases you will need the opinion of a physician
Now coming to swelling in just one ankle could be due to 1) joint arthritis, 2) tendon strain, 3) ligament sprain.
Whatever the reason, rest, crepe bandage, activities as tolerated, in case of pain mild analgesics like paracetamol.
Next Steps
will need more information to give a more specific treatment plan