update some elements in the array

Hello,

Could someone please tell me that is there anyway that I can update only
a few elements in a Postgres array.

eg; I have an array of text, declared as 'text[]'.

{"1","2","3","4","5"} ==> {"1","2","7","8","9"}

I want to update the last three element without affecting other
elements. I could have done it by retrieving all elements in the array
first and the do a

UPDATE table
SET arr='{"1","2","3","4","5"}'
WHERE blahblah

however, that might be troublesome, so I tried

UPDATE table
SET arr[1]=val1
arr[2]=val2...

but it didn't work. If anyone knows, please tell me. Thanks for the
help.

- Bernie