If you're asking how to do this, it sounds to me like you don't know much about Active Directory, so if I were you I wouldn't touch it. I'd find someone else who knows what they're doing.
What specifically do you want to change? Do you just want to change the account name, or something else?
Say you have John D. Smith and his username is JSmith but company policy wants you to change to to SmithJD, just change the account name in AD.
You want to make sure that nothing is running as that user though (scheduled task, computer service, etc. - it shouldn't be anyway because that's bad practice, but that could happen) otherwise the job/service will try to run as that user and continuously lock their account out.