how do you convert the date format from MM/dd/yyyy hh:mm:ss tt to dd/MM/yyyy hh:mm:ss tt

You were trying to parse with the format you wanted, not the format you have!
Try:

Dim dDate As DateTime = DateTime.ParseExact(str, "MM/dd/yyyy hh:mm:ss tt", CultureInfo.InvariantCulture)

DateTime variables do not contain month first or day first type data. They contain numeric value that represents the date and time. When converting from DateTime format to String format, a "format" string is included to tell the Format function how to create the resulting string.

ParseExact requires that the number of characters match exactly. Parse allows single digit month or day.

Dim str As String = "6/26/2013 10:33:21 AM"
Dim dDate As DateTime = DateTime.Parse(str, CultureInfo.InvariantCulture)
Dim strDayFirst As String = Format(dDate, "dd/MM/yyyy")

Tested Visual Basic .NET 2012

Single "M" is not a known format code: try double

Dim dDate As DateTime = DateTime.ParseExact(str, "dd/MM/yyyy hh:mm:ss tt", CultureInfo.InvariantCulture)