How to provide connection string dynamically.

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ASP.NET

MVC

How to provide connection string Dynamically.
I have connection string name but I want to set Database name in appsetting.json file at run time.

What I have tried:

string connetionString;
                   SqlConnection cnn;
                   connetionString = @"Data Source=.;Initial Catalog=Name;Integrated Security=True";
                   cnn = new SqlConnection(connetionString);
                   cnn.Open();

                   SqlCommand command;
                   SqlDataAdapter sqlDataAdapter = new SqlDataAdapter();
                   SqlDataReader sqlDataReader;
                   string sql, Output = "";

                   sql = "select * from table";
                   command = new SqlCommand(sql, cnn);