Can you cast a spell with flash during your opponent's upkeep before they untap their lands?

The turn structure places the Untap Step first, followed by the Upkeep Step (CR 500.1, 501.1, 502.1). Crucially, no player receives priority during the Untap Step — permanents simply untap and the step ends (CR 502.4). This means you cannot cast any spells, including flash spells, before untapping occurs.

Flash allows you to cast a spell any time you could cast an instant (CR 702.8a). Instants — and flash spells — can be cast when you have priority. Priority is first given to the active player at the beginning of the Upkeep Step, but by that point the Untap Step has already fully resolved and all lands are already untapped (CR 116.3).

So while you absolutely can cast a flash spell during your opponent's upkeep step (a common and powerful play), their lands will already be untapped when you do so. There is no game state where you hold priority with untapped lands still needing to untap in their turn.

Example: Your opponent starts their turn. Their lands automatically untap (no priority given). Then upkeep begins — you pass priority and they receive it. Now you may cast your Aether Flash or similar flash permanent, but their lands are already untapped.

If you want to interact before an opponent has access to their fresh mana, the only window is during the End Step of the previous turn, but even then lands untap at the very start of the next turn before anyone gets priority.