If you reschedule for a legitimate reason, this won't be viewed negatiely, as long as you manage the process properly. However it's not ideal to say the reason to reschedule is to have more time to prep, so I'd suggest you tell a bit of a white lie and say you have a personal situation (without giving details) which means you can no longer attend the session next week. My suggestion would be:
1) Contact HR, saying that something personal has come up and that you can no longer attend the current interview date
2) be flexible and very appreciative of HR as they try to reschedule you for a different session
3) Of course, if they tell you you cannot reschedule, tell them you will try find a way to still attend (you obviously still want to attend the interview, in case they can't reschedule).