Stand on a step with just your toes and let you weight into your heels. This stretches the calf muscles
My heels were hecka stiff so my trainer made me stretch my heels down until they burned. Practicing like that helps too, I saw a big improvement just the next week of doing that
Walking around on your heels is great it stretches the muscles in the backs of your legs
Some people at my barn will flat in ankle weights with the weight part to the back of the ankle
I use a calf stretcher, it's a tool runners use. You can get it on Amazon for $20 or less.
In addition, while you’re riding, do tons of two point! It not only makes you stretch your heels, but also works
Yoga! Downward dog pose gets those muscles loose so they can stretch. It really works. And doing the stretches on the stairs like Marissa said. My fav is Tara Styles. You can do her videos from YouTube.
Stand on the edge of a ledge or stairs and stretch down into your calves and heels