POSIX shell does NOT have array types. However, with a bit of inefficiency, you can get array-like semantics
The trick is that you do have one and only one array - the positional parameters $1 $2

Replacing the contents of the $@ array is easy:
--
set -- a1 b2 c3
printf "%s" $@

Or, perhaps more usefully:
set -- *	# list current directory file names
printf "%s" $@

another test. usage: ./cmd a b c d
--
orig_array=$@
echo $1

set -- foo bar baz boo
echo $1

eval "set -- $orig_array" # http://www.unix.com/man-page/posix/1posix/eval/
echo $2