This is a little tricky as the best way to do this is to take the varchar convert it into a datetime and then format it. Annother complication is that the format you want is not a format that SQLServer will output.
So.
SELECT CONVERT(DateTime, DateOfBirth) from people
will get you the date time and we can then convert it to a string format as follows
SELECT CONVERT(DateTime, DateOfBirth), 106) from people
this will produce the string output 'dd Mon YYYY'
then its just a matter of replacing the spaces with '/'
SELECT REPLACE(CONVERT(varchar, CONVERT(DateTime, DateOfBirth), 106), ' ','/') FROM people
will get you the format you want.