• In the Toolbox, select the Background Eraser Tool.
• Choose brush size and style*.
• Choose in Layers palette Background.
• Erase.
• The gadget Sampling on the Options bar allows to select one of three erasing modes:
  • Continuous (default method) allows to erase everything.
  • Once allows to erase only color you first click; for example if you start a stoke on a red area the tool erases only red pixels.
  • Background Swatch allows to erase only pixels containing current background color.
• Tolerance gadget sets the tool's sensitivity.
• Limits defines range of the tool: all over image (Discontiguous), only adjacent areas (Contiguous) or only inside pixels (Find Edges).
• Protect Foreground Color option disallows to clear pixels containing current foreground color.

* More about choosing brush.