Broken Lip on Battery Casing of a Timex Chrono Watch?

I have a Timex Chrono Alarm wristwatch, and the battery has recently died. I went to WalMart where I bought the watch (roughly a year prior) and bought the appropriate battery (2-pack) and a new watch band, and asked the old lady behind the counter if she could replace the battery for me. She told me she couldn't do it because the watch wasn't purchased here, but I told her I HAD bought the watch there, it had just been a year prior. After several failed attempts to get the back off myself (the "lip" appears to be fractured, making it extremely hard to remove) I'm left extremely frustrated. Is there a better way to remove the back from the watch so I can replace the battery? Or should I just go back to walmart and b*tch at the old lady until she does it?