Changing The Loop on WordPress hosted blogs?

Hi,
I’m a newbie. I set up a word press blog (hosted on wordpress.com) with one of the standard templates.
My client wants the posts to show for the current month only. I see that one can make changes to The Loop to fix this, but can you do that if you don’t have your own installation, and then customized template?
If you can, then how do you access the code to the index.php page?
Thanks,
JW