"Can I change my arrow head style in Civil 3D?" I had this question come up recently, and the short answer is sort of. In order to create a custom arrow head style within a Civil 3D label, we need to choose an arrow head style that is currently not being used.
From the list of arrow head styles, we can choose an arrow head style such as box filled. After we have chosen this style, we can simply hit APPLY and OK to get out of the label style composer. Now we can run the block command as seen below and redefine the _Boxfilled block to whatever object we select. In the screen capture below, I have chosen a helix.