assign goto url to all buttons

Something like this? This code inspects all buttons in the document and gathers the ones with a single gotoURLBehavior. It shows the list in a simple dialog, where you can also enter a new URL. Press OK to change the listed URLs, or Cancel to bail out.

If the dialog shows other URLs than the one(s) you want to change, you'll have to devise a way to discern these. (Although another way could be to add checkboxes for each one to change, but that's a bit more work )

allButtons = app.activeDocument.buttons;
urlButtons = [];
urls = [];
for (i=0; i<allButtons.length; i++)
{
  if (allButtons.gotoURLBehaviors.length == 1)
  {
    urlButtons.push (allButtons);
    urls.push (allButtons.gotoURLBehaviors[0].url);
  }
}
urlDialog = app.dialogs.add ({name:"URL buttons",canCancel:true});
with (urlDialog)
{
 with (dialogColumns.add())
 {
    for (i=0; i<urls.length; i++)
      staticTexts.add ({staticLabel:urls});
    edBox = textEditboxes.add({editContents:"http://www. ...", minWidth:240});
 }
}
if (urlDialog.show())
{
  for (i=0; i<urlButtons.length; i++)
    urlButtons.gotoURLBehaviors[0].url = edBox.editContents;
}