how to change the format of datetime in c#

First of all - DateTime stores it's value in binary and independent of visual representation (format), however it has a ToString() method, that can format the value as you wish: http://msdn.microsoft.com/en-us/library/zdtaw1bw(v=vs.110).aspx[^]

if you have datetime in string format like "12/05/2014 12:17:20" then better convert this to a DateTime type. DateTime can be convert to string format by giving the format which we needed.
for the String to DateTime conversion use DateTime.ParseExact[^] or DateTime.TryParseExact[^] methods
When you have the DateTime object, call the ToString method with custom format string as you need. Check Custom Date and Time Format Strings[^] for more information.

Sample code:

DateTime dt = DateTime.ParseExact("12/05/2014 12:17:20", "MM/dd/yyyy hh:mm:ss", CultureInfo.InvariantCulture);
string result =dt.ToString("dd MMM yyyy hh:mm:ss");

string dateStr = DateTime.Now.ToString("ddd dd MMM yyyy HH:mm:ss");