Are you looking to change the underlying value of the cell? or just the way it's presented?
Normally, if you want a date shown as mmm-yy you can do that through cell formatting, where you can define which elements of a datetime cell you want to show (and is, if I remember correctly), essentially what Excel's TEXT() function does - takes a value and a formatting string to define how to show the value.
Failing that, in the absence of the TEXT() function, you can construct it yourself using text functions like:
=LEFT(MONTHNAME(MONTH(A2)),3)&"-"&YEAR(A2)
(Assuming A2 has a date value)