how to access master page control from content page

In Content page you can access the label and set the text such as

Here 'lblStatus' is the your master page label ID

Label lblMasterStatus = (Label)Master.FindControl("lblStatus");

lblMasterStatus.Text  = "Meaasage from content page";

In the MasterPage.cs file add the property of Label like this:

public string ErrorMessage
{
    get
    {
        return lblMessage.Text;
    }
    set
    {
        lblMessage.Text = value;
    }
}

On your aspx page, just below the Page Directive add this:

<%@ Page Title="" Language="C#" MasterPageFile="Master Path Name"..... %>
<%@ MasterType VirtualPath="Master Path Name" %>   // Add this

And in your codebehind(aspx.cs) page you can then easily access the Label Property and set its text as required. Like this:

this.Master.ErrorMessage = "Your Error Message here";