Just select an area with Photoshops choice of tools- click & drag the mouse to make a selection and the use the 'paintbucket' tool to fill with whatever colour you want....
The 'proper' way to do it involves using 'layers' but you don't need them for a simple operation.
Use the lasso tool to draw round the relevant area. (Optional but recommended: Right-click and select 'feather'. Try 5 or 10 pixels as an initial setting).
Then either use the technique suggested above or try the options under Image > Adjust
It's all in the help section!
Too long-winded to explain here .. but lassoing and stiching/clicking a line around object to enable cutting/pasting of colour or another part of image/different image.
If you want to select an area which is distinct from the rest of the image try using the magic wand or quick selection tool.
Also, if you need to select another area (without losing the first selection), hold down 'shift' when you make the new selection.