This video knitting tutorial will help you learn how to rip back your stitches, or frog your project. Some times mistakes are made that can not be fixed without ripping out your stitches to that point in the project. This video shows a trick for doing this without risking ripping back too far.
Take a knitting needle or a darning needle attached to some waste yarn and thread it through the front leg of each stitch in the row below your mistake. Then pull out your knitting. Once it reaches the needle it should stop.
If you accidentally put your needle through the back leg of the stitch, just knit through the back loop once you work that stitch, which will straighten it out.