Use Layer › New Adjustment Layer and pick an adjustment. The change sits on its own layer above your image, so your original pixels stay as they were and you can edit or hide the adjustment later.
Add an adjustment layer
- Open your image and click the layer the adjustment should sit above in the Layers panel.
- Choose Layer › New Adjustment Layer, then an adjustment such as Curves…, Levels…, Vibrance… or Hue/Saturation….
- Change the settings in the Properties panel, which shows the settings of the new layer.
The same list is behind the button at the bottom of the Layers panel, labelled Create new fill or adjustment layer when you hover over it.
The adjustments in the menu are Brightness/Contrast, Levels, Curves, Exposure, Vibrance, Hue/Saturation, Color Balance, Black & White, Photo Filter, Channel Mixer, Color Lookup, Invert, Posterize, Threshold, Gradient Map and Selective Color.
Limit it to one area
- Select the area first, for example with the Elliptical Marquee Tool. See Select part of an image and mask it.
- Add the adjustment layer as above.
- With the adjustment layer active, choose Layer › Layer Mask › Reveal Selection. The adjustment now shows only inside the selection.
Change it later
Click the adjustment layer in the Layers panel and edit its settings in Properties. Every step you take is also listed in the History panel.
Apply an adjustment straight to the pixels
If you do not need to change it later, use Image › Adjustments, for example Image › Adjustments › Curves…. These change the pixels of the layer directly and are not kept as a separate layer.