You can toggle the visibility of a layer by touching the small "tick" by that layer.

Remember that different frames don't share the visibility of their layers with other frames. E.g., if you change the visibility of the 3rd layer of a frame, the visibility of the 3rd layer of other frames will not be touched.

It's also good to remember that hidden layer is also hidden when exporting to png, sprite sheet or gifs. It's there for convenience and is the expected behavior, not a bug! Fear not, if you save your project as a .anim, your hidden layers (even though they're hidden), they'll be saved, and next time you open your project, they'll be there (with their visibility setting as it was when saved)