Slender: The Arrival's recent 10th anniversary update made a ton of updates to the game, including visual changes and new chapters. One thing that has remained the same from the original - aside from some incredible updates in the new graphics engine - is the game's secret Memories level, which players will be able to access by finding a hidden collectible.
Here's how to access the secret Memories level in Slender: The Arrival after the game's 10th anniversary update.
Slender: The Arrival: How To Access Secret Memories Level
To access the secret Memories level, you'll need to start the Flashback level. (You can either do this during your first playthrough or access the Flashback level from chapter selection if you've already beat it.)
You'll start out in a covered building; walk out of it to see that you're right in front of an enormous forest. From here, keep walking out and you'll see a gate on your left. Stay on that side of the path, where it meets the cliff that looks like a huge drop (don't worry, you can't fall down).
Keep an eye on the trees at the edge of the path where it meets the cliff; you'll eventually find a tree with a small teddy bear at the bottom of it. This may take a bit of searching, but rest assured - that creepy teddy bear is hiding out there somewhere.
Once you find the teddy bear, simply interact with it. Your character will pick it up and take a look at it. You'll then be met with a loading screen before you're transported to the Memories level, which will have you on a beach. From there, it's pretty straightforward as to how to complete it; ignore your parents' calls as you pick up toys on the beach and stray further and further.