C# Help me with Prepare Statement | Update

C# Help me with Prepare Statement | Update

Can you help me with prepare statement?. I using this code which you can
watch down but when i used this code so prepare statement still send me
problem with @reference already exist but i want only update existing
data.
// Create and prepare an SQL statement.
command.CommandText = "UPDATE table SET column = @nazev " + "from table
left outer join next.table on next.table.column = table.column " + "WHERE
table.column = @reference";
SqlParameter referenceParam = new SqlParameter("@reference",
SqlDbType.VarChar, 30);
SqlParameter nazevParam = new SqlParameter("@nazev", SqlDbType.VarChar, 100);
// I get data to list from richtextbox and from richtextbox.Rows i added
data to lists
referenceParam.Value = Definitions.ReferenceSubjektu[i];
nazevParam.Value = Definitions.Preklady[i];
// Command for Název
command.Parameters.Add(referenceParam);
command.Parameters.Add(nazevParam);
// Call Prepare after setting the Commandtext and Parameters.
command.Prepare();
command.ExecuteNonQuery();