How to Access controls on Master Pages from Content Pages in ASP .NET

We will assume that we have a Label control called "Label1" on your Master Page, you might want to change the text of that Label.
To do that write this code in your content page:

Label lbl = new Label();
lbl = (Label)Master.FindControl("Label1");
lbl.Text = "Text";

Of course, this is all assuming the label is OUTSIDE the ContentPlaceHolder

Source: https://www.nilebits.com/blog/2003/05/how-to-access-controls-on-master-pages-from-content-pages-in-asp-net/