Add a new adjustment layer from the Adjustments tab (use levels)
This add a bunch of features to your pallet
1. it adds the levels adjustment as a layer, as you can see from the thubnail. By clicking on this thubnail you will aways be able to change these ajustments.
2. to return to the other ajustments in the tab click the arrow on the side.
3. to hide this ajustment (without deleating it) click the eye
4. to temporaraly hide the last move you performed in the ajustment use this eye, or the circular arrow to revert.
5. you can toss this adjustment with the trash can.