Is the date saved as text? If the date is saved in the Excel standard, then it's just a number, and you just choose what format you want to see it displayed in. If it's text, do you want the result as text, or as an Excel date? Can you always expect a 4-digit year, or will there sometimes be 2-digits?
This should work for text values:
=SUBSTITUTE(MID(A1,FIND("/",A1)+1,2)&"/"&LEFT(A1,2)&MID(A1,FIND("/",A1,4),5),"//","/")
Depending on your regional settings, you can wrap that in DATEVALUE to get an Excel date.