Swelling in the feet and ankles can be from a number of things. Often it is from an injury, but it can be an injury at your foot, ankle, knee, hip, or even back since the fluid or swelling will often follow gravity.
If you have not had an injury to your feet and/or ankles, and you are having swelling, make sure you check in with your doctor because it could be a symptom of a more serious issue.
Ankle pumps, calf stretches, and straight leg raises (SLR) are great ways to help reduce swelling in your feet and ankles. Working the calf, quads, and hip flexors work as natural pumps to move the fluid out of your legs.
Also, propping your leg up above your heart and just relaxing can help reduce the swelling as well.
Some seated exercises like seated hip flexion, seated long arc quads (LAQ), and heel and toe raises are great to do if you have to work at a desk all day. You can also do the heel raises standing for a tougher exercise.