I've had it about a month and a half.
How do I go about returning it?
I've been using a Keeper for about 3 years, after discovering their existence while reading papers at work -- I work in medical research). I love it, for various reasons. (Weird personal reason: being a little strange, I always used to worry vaguely that in the event of World War 3, supplies of tampons would run out in the post-apocalyptic world. Now I have my cup, that worry is over. :-)
Actually, I used a Keeper for about 2 1/2 years, before it was damaged by an Act of Stupidity. As a replacement, I bought a Mooncup, which I found out about through the Museum of Menstruation site. That has also proved extremely satisfactory. Personally, I might even say I prefer the Mooncup, but I'm sure that preference varies between users.
Anyway, nice to see a Keeper community. I do my best to evangelise menstrual cup usage, but it's discouraging. Most women are not receptive to the idea, unfortunately.