If you had initially pleaded guilty but at a later point in the process wished to change your mitigation plea (i.e. from pleading guilty to pleading not guilty), the Singapore Courts will not permit such a course of action.
However, it may be permitted in cases whereby, if the accused has managed to show that there are valid and sufficient grounds that satisfy the Court, the accused may then retract the plea, such as providing ample evidence to prove that the earlier plea was invalid/unclear. If the retraction of the guilty plea is successful, the Court will then send the matter back for trial.