Then welcome to the VP Community!
What are you trying to achieve here? The reason I ask is because normally the Alt fragment doesn’t have any numbers assigned to it, but the messages you have added on the other hand do.
You can check this for yourself by selecting the Alt fragment and then press enter or right click and select the “Open specification” option listed in the menu. Also see:
But to answer your question: You can’t change the number sequence because that would invalidate your diagram. What you could do is hide parts of it. For example hiding the caption or the arguments. You can do that by right clicking on one of the messages, find the “Presentation option” and in there you’ll find multiple options to enable or disable.
I hope this helped!