string rawConnString = Properties.Settings.Default.ConnectionString;

rawConnString.Replace("<<DATA_SOURCE>>", server);
rawConnString.Replace("<<INITIAL_CATALOG>>", "tempdb");

string finalString = Properties.Settings.Default.ConnectionString.Replace("<<DATA_SOURCE>>", server).Replace("<<INITIAL_CATALOG>>", "tempdb");

var finalConnString = rawConnString.Replace("<<DATA_SOURCE>>", server)
    .Replace("<<INITIAL_CATALOG>>", "tempdb");

string tempString=rawConnString.Replace("<<DATA_SOURCE>>", server);
sting finalstring=tempString.Replace("<<INITIAL_CATALOG>>", "tempdb");

string rawConnString = Properties.Settings.Default.ConnectionString;

string finalConnString = rawConnString
                            .Replace("<<DATA_SOURCE>>", server)
                            .Replace("<<INITIAL_CATALOG>>", "tempdb");

string connString = Regex.Replace(
    rawConnString
,   "(<<DATA_SOURCE>>)|(<<INITIAL_CATALOG>>)"
,   m => m.Groups[1].Success ? server : "tempdb"
);

finalstring=rawConnString.Replace("<<DATA_SOURCE>>", server);
finalstring=finalstring.Replace("<<INITIAL_CATALOG>>", "tempdb");

finalstring=rawConnString.Replace("<<DATA_SOURCE>>", server).Replace("<<INITIAL_CATALOG>>", "tempdb");