You can't actually retract a statement. Your original statement could still be used in court, with you then being called upon to explain why you've changed what you said. (If it appears that you're trying to prevent an offender from being found guilty of an offence you could be prosecuted for attempting to pervert the justice. That only happens very rarely but, for example, there has been a case where a woman was sent to prison for refusing to give evidence against the man who raped her. So a certain amount of caution is needed when changing the evidence that you do/don't provide to the police, the CPS and the courts).
You can, however, indicate that you don't want an offender prosecuted but the CPS is not obliged to go along with that. (The CPS policy is not to drop charges in relation to 'domestic' incidents simply because the victim has changed their story. That's to prevent offenders from intimidating their victims).
You're also entitled to provide the court with a statement indicating the impact that an offence has had upon your life. While such statements are normally used to stress the trauma experienced by victims of crime, there have been cases where the victim has indicated to the court that the offending has had very little impact upon them and that they do not wish to see the offender severely punished).