Here's how to export a PNG of any frame in an artboard, directly from the editor:
Right-click on the frame you want to export
Select Export as PNG from the menu
The image will download instantly to your device
This also works across multiple artboards at once: just select them all before right-clicking.
If you need a frame-by-frame export rather than a single still, check out our article on exporting a PNG sequence, which is available on the Max plan.