Use the DateTime.SpecifyKind static method.
Creates a new DateTime object that has the same number of ticks as the specified DateTime, but is designated as either local time, Coordinated Universal Time (UTC), or neither, as indicated by the specified DateTimeKind value.
Example:
DateTime dateTime = DateTime.Now;
DateTime other = DateTime.SpecifyKind(dateTime, DateTimeKind.Utc);
Console.WriteLine(dateTime + " " + dateTime.Kind); // 6/1/2011 4:14:54 PM Local
Console.WriteLine(other + " " + other.Kind); // 6/1/2011 4:14:54 PM Utc