Do you mean the shifters for drop handlebars? If so, the shifters have a little paddle behind the brake lever.
Push this a short distance and it'll let out a bit of cable (i.e. shift into a smaller sprocket or chainring).
Push the same lever over further, and it will take in cable (i.e. shift into a larger sprocket or chainring).
Sounds confusing, but when I tried it out it was fairly intuitive in use.