Change Master Page control dynamically from Content Page

I want to change Master page control dynamically from Content Page So I used FindControl as below:
Label masterlbl = (Label)Master.FindControl("lbltotal"); And I want to assign some value to it dynamically. It is working fine on page load. But when I am trying to assign value on Dropdown SelectedIndexChange, It does not change value.
So how to do it ?
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