Short answer. Both are possible. Rule 124, Section 15 lets the Court of Appeals either conduct the new trial hearing and receive evidence itself, or instead refer the trial back to the court of origin. The choice belongs to the appellate court alone; neither the prosecution nor the accused can compel it to pick one route over the other.
What the law says
When a new trial is granted, the Court of Appeals may conduct the hearing and receive evidence as provided in section 12 of this Rule or refer the trial to the court of origin.
Rule 124, Section 15 — Where new trial conducted. Read the full provision →
A choice, not a fixed rule of remand
Once a new trial is granted, Section 15 does not automatically send the case back to the trial court. It gives the Court of Appeals two paths: conduct the hearing and receive the evidence itself, following the evidence-reception procedure set out elsewhere in the same Rule, or refer the trial to the court of origin instead. Which path is taken is left to the appellate court's own judgment on the facts of the case; the rule does not require it to justify choosing one route over the other, and neither party has a right to insist on a particular outcome. Section 15 does not itself set a deadline for completing the new trial once the path is chosen, and it does not distinguish between a new trial granted on the accused's motion and one initiated by the Court of Appeals on its own; the same two options apply either way.
Handling it at the appellate level
If the Court of Appeals chooses to conduct the new trial itself, it follows the same evidence-reception procedure the Rule prescribes for new trials generally, rather than improvising its own process. This keeps the new-trial hearing tied to an established mechanism even when it happens at the appellate level rather than back at the trial court. In practice, that can mean the appellate justices themselves hear witnesses and receive documentary evidence, folding what might otherwise be a separate trial-court proceeding and a later appellate review into a single forum.
Sending it back down instead
Alternatively, the appellate court may refer the trial to the court of origin — the same trial court that rendered the original judgment. This route may make sense where witnesses, evidence, or logistical considerations favor conducting the new trial where the case originated, rather than at the appellate level. Either way, the choice belongs to the Court of Appeals rather than to either party, since Section 15 frames both options as available to that court alone. Both the prosecution and the accused remain bound by whichever route the appellate court selects, since Section 15 vests that choice in the court alone and gives neither side a veto over it.