There are lots of ways to do that. One of the easier ways is to use the clone tool to clone certain parts of the image and paint right over the parts that you want erased. For example, if you have a background that's filled with grass but on one area, there's a dog poop and you want to erase that poop, you would take the clone tool, hold down alt to define what area of the image you want to clone (in this case, a part of grass) and then start drawing over that poop. That will write over the dog poop with the grassy area that you defined to clone. Another example of some other technique is to use the dodge or burn tool to burn or dodge an area with a certain threshold level. This requires trial and error. Another way to change a certain color within a whole image (like change all the reds in this image to yellow or change it slightly etc.) is to use the selective color tool.
Basically there's almost an infinite amount of ways to do something in PS so learn everything about photoshop and use that head of yours. I've been doing photoshop for 8 years and it pays me $50 an hour : )