Just stretch your heels on a stair or aerobics 'step'.
I've cobbled a few pictures from the internet to illustrate what I mean. Do this for 2 minutes every day, preferably twice. Keep doing it. Will take at least 2 months to fix, possibly up to six (in my case it was 2 months).
Stretch your calf AND your Achilles. Bend the knee to stretch the Achilles, and don't bend it, in order to stretch the calf. Hold your weight on the ball of the foot, and let the weight go into the back of the leg.