Hello servet.zeybek,
Thanks for contacting us with your question.
Unfortunately, there is no API option that could change the milestone shape. However, you could do it with a wrap. Here you can find more informations about it:
https://www.highcharts.com/docs/extendi ... highcharts
Here's an example of a such wrap:
https://jsfiddle.net/BlackLabel/52esbm6p/ To use it simply set
customSymbol to one of available shapes:
diamond, circle, square, triangle, triangle-down
Btw. I think that API property like this would be very usefull and If you want to see it as an officjal API option, I encourage you to create a Feature Request on our github.
Let me know if that was what you were looking for!
Best regards!