  Volume entropy is an important invariant of metric graphs as well as
Riemannian manifolds. In this note, we calculate the change of volume entropy
when an edge is added to a metric graph. Using the first result, we investigate
the change of volume entropy when a vertex and edges around it are added.
  In the second part, we estimate the value of the volume entropy which can be
used to suggest an algorithm of calculating the persistent volume entropy of
graphs.
