How can I replace my column values with new column values in SQL? Asp.net C#

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C#

ASP.NET

SQL-Server-2012

Hi.
I'm replacing my old column values with new column values but my query is not executed and sometime give me error "invalid 'tbl_field' " but tbl_field is present,tbl_field is my table name.How can i resolve this issue.

What I have tried:

Here is my code aspx.cs

public void updateTbl(string newname)
    {
        string oldname = Session["oldname"].ToString();
        String db = Session["value"].ToString();
        SqlConnection cnn = new SqlConnection("Data Source=HAMEED_KHAN\\SQLEXPRESS; Initial catalog=db_compiler; Integrated security=true");
        string RNquery="USE "+db+" EXEC sp_rename '"+oldname+"', '"+newname+"'";
        string updateQuery = "USE "+db+" Update 'tbl_field' SET Table_Name= replace(Table_Name, "+ oldname + ", " + newname + ")";
        SqlCommand cmd2 = new SqlCommand(updateQuery, cnn);
        SqlCommand cmd = new SqlCommand(RNquery, cnn);
        cnn.Open();
        cmd2.ExecuteNonQuery();//Here ERROR 'incorrect syntax near tbl_field
        cmd.ExecuteNonQuery();
       
        cnn.Close();
    }

My 'RNquery' which is used to change table name in database working fine but issue with 'updateQuery' its give my error but when i execute this query in sql ,it's working fine,i think there is some little mistake with ',or " etc.Thanks