Kicking with your toes down toward the bottom of the pool is like swimming with a hand brake on, so if you want to reduce some of the drag component of your kick then improving ankle flexibility is a great way to do this.
What to do?
A way in which you can improve this is to sit on your knees, put a towel under your ankles (more than one if needed) and over time drop more of your weight onto the ankles and then progress to remove the towels as support and then and leaning back onto the ankles.
When to do this?
Do this over a number of weeks for small amounts of time. People with low flexibility 5-15sec might be all you can handle so you might want to do this quite regularly and build this up to a point where you could do this for 3x30-60sec.
Have the support you need easily accessible so you can do this while you are watching TV, playing with your kids on the floor, having a read or even eating.
Don't do this if you are feeling an unusual amount of pain and discomfort, consult a physio or other professional for specific advice on what you should do.