In "Animal Crossing: New Horizons," the change in season is marked by gradual shifts in the environment. While it technically becomes fall on September 1st in the game, the visual changes, such as the ground becoming brown and trees transitioning to autumn colors, typically occur over the course of September.
You should start to see the fall colors and changes in the scenery as the month progresses. If you're not seeing any changes by mid-September, it might just take a little longer to fully transition. As for summer shells, they will still appear on the beach until the end of the season, with fall shells becoming available later in September. Enjoy the fall festivities when they arrive!