One of the more quirky aspects of Nintendo's Animal Crossing series is how the games like to hide certain information from players. Instead of outright telling you how to unlock different tools or invite certain villagers, you'll often just stumble upon the solution while going about your daily routine. It's kind of like how life works: there's no manual for living a "good" life.
Still, players that have been tinkering with New Horizons, the latest installment in the series, have discovered some tricks that may make certain events trigger. One of those events happens to be the upgrade of Nook's Cranny, your island's retail store. Run by Tommy and Timmy Nook, this building will undergo a series of renovations to increase its size and provide players with more goodies to buy daily.
While we haven't discovered how many upgrades there are, we do know the steps needed to achieve the first upgrade. If you want a bigger Nook's Cranny, read on for how to achieve that.
First things first: be patient. Unlike previous entries in the series, Nook's Cranny won't upgrade simply by spending money at the store. That is still a pre-requisite, but the biggest factor is time. After 29-30 days of being an active resident on your island, the store will automatically close and renovate into a bigger version. You can speed the process up by time skipping, but you'll need to change your system's clock by a single day and then relaunch the game. You can't just do all 29 days at once.
The second step is related to bells. You'll have to accumulate 200,000 bells spent/earned at the store. This means you spend a lot of money and sell a lot of items to the Nooklings. You'll likely have this without even trying, but you can just visit once a day and buy some random things to make sure you have it.
Lastly, Mable of the Able Sisters needs to have visited your island at least once. You don't need to unlock the Able Sisters' store, but this prickly hedgehog must be at your island at least once to upgrade Nook's Cranny. You can spot her near your Resident Services Building on random days selling some goods. Talking to her is key to unlocking the store, so if you have that, you're all good.
Once you've met all these conditions, your store should upgrade on its own after the required time has passed. We're not sure if future upgrades will have the same steps or not, but it seems Nintendo packed a lot more waiting into New Horizons. That's perfectly fine as we all deal with self-isolation.