Change your master page file property

if you want to change a maser page content according to your content page then you just write code in the preinit event within the content page codeview


 protected void Page_PreInit(object sender, EventArgs e)
    {
     
        if (some condition)
        {

            this.MasterPageFile = "~/MasterSetup/AdminErst.master";
            //Image img1 = (Image)Master.FindControl("logo");
            //img1.ImageUrl = "~/image/ErstwhileLogo.png";
           // Label lbl = (Label)Master.FindControl("AdminPNPLBL");
            //lbl.Text = "Erstwhile Infrastructure PrivateLimited";
            //img1.Width =178;
           
        }
else
{
 this.MasterPageFile = "give your anotherFilename";
}
    }