Don't take this the wrong way but renaming the home folder requires a lot of confidence in what your doing.
Your changing ownership permissions from the root of the system and if not followed to the letter or if unexpected issues arise (who knows what's happened on a pre owned machine) you can end with a very unstable or non-functional system.
Instructions
here
Even once the home folder has been changed you'll still have apps and accounts in the previous owners name. You can't change the ownership/Apple ID on applications the previous owner has purchased on the App Store.
I would strongly suggest a clean install. I can provide instructions if required.
However, whichever way you go you must have a backup. Whether that's a full disk image, a time machine backup or just copying your irreplaceable data off the machine manually. I'd recommend a regular backup anyway but making any kind of wholesale system changes like this it's essential. I'd also suggest have an OSX 9 USB installer is good practice too.
Sorry to sound so preachy.