We can access master page controls from the content pages.For examples :-
TextBox txtbx = (TextBox)Master.FindControl("txtMaster");
txtbx .Text = "DotnetFunda";
In the above code, FindControl is a method which is taking name of the control which is in the master page. Then it is casting in the textbox. Now, with the help of the text property we can assign any property.
Thanks and Regards