hi,
withdrawing any statement in criminal / civil proceedings is a serious step and should be considered very carefully. assuming that the statement was made for a criminal trial the police would want to know why you are retracting your statement. are you being intimidated? threatened? if so this is a very serious matter and the police do have the powers to deal with this.
a retraction can be made at anytime. it can be done by visiting the police station (its always best to make contact with the officer / dept which dealt with the original compliant) or contacting the witness care people (assuming you have been warned to attend court).
HOWEVER, just because you have retracted it does not necesarily mean that this is the end of the matter. dependant upon who you are (victim of domestic violence, the victim of a serious assault or just someone who is not a primary witness) and what the offence is and who the suspect is or has done will have a baring on the matter.
the courts can choose to continue in your absence or they can if they choose to summons to to court (you can be arrested and taken there if necessary) you give your evidence.
whatever you choose to do think very carefully and speak with the police / witness care before you make your decision.