Just kick as high as you can a lot each time you train. Stretch properly afterwards. With time it will be easier and easier to kick high.
Kicking high is not just about being fleixble. In fact, you don't need to be all that flexible to kick high. Reaching the required flexibility only takes a few months with proper stretching. Most of it is proper body mechanics, some strength, and control over your muscles, so that they all work together in harmony to get your shin up there with some speed. And that comes with kicking high lots of times.
I used to work the bag for hours at the time, trying to mimic better fighters execution of techniques.