Your Achilles tendon runs from the back of your knee down to the back of your heel. You can locate the tendon just behind your ankle. A tear or break in the Achilles tendon almost certainly requires surgery, so you should strengthen this part of your body.
Sit in a chair with your legs up on a padded stool in front of you. Or preform the exercise laying on a couch or bed.
Point your toes while slowly moving your feet downward. You will be flexing your calves, which causes the Achilles tendon to engage. Only go as low as you can without causing pain.
Bend your ankles and lift your toes back up toward you. This will help stretch the tendon. Flex your feet back toward your legs as far as possible. This exercise doesn't need to be painful to be effective.
Continue the motion slowly, repeating the pointing and flexing for as long as you can. You can start with 30 seconds of movement and increase each session by 30 second increments.
Change the movement of your ankle for a variation on this Achilles exercise. Instead of performing an up-and-down motion, switch to turning your toes in circles. Little circles are fine, but do them as long as you can without causing any pain. Switch directions of the circles every 30 seconds.