Graphviz: how to set ‘default’ arrow style?

Use the edge attribute statement, as stated in the DOT Language documentation.

digraph graphname {
    subgraph clusterA {
        node [shape=plaintext,style=filled];
        edge [arrowhead=normal,arrowtail=dot];
        1 -> 2 ;
        2 -> 3 -> X2 -> 5;
        6;
        7;
        label = "A";
        color=blue
    }
}