Question

There is a aspx page with Master page.
This master page has a usercontrol.

I use script manager in the aspx page. I was tyring to update label text in usercontrol (Master page) after click a button.

Answer 1

In the page you can do something like this:

WebUserControl1 wuc = (WebUserControl1) Master.FindControl("wucId1");
wuc.ucLabelText = "whatever";

where WebUserControl1 is the name of your user control and wucId1 is its ID in the master page.
Assumed you have in the user control an attribute named ucLabelText for the label’s text