Jelqing, or stretching your penis in an attempt to make it longer, is a very bad but popular idea. It is highly inadvisable. The penis is comprised of three spongy masses (corpus cavernosa) stacked that become saturated with blood during arousal. As such, there are millions of tiny blood vessels and capillaries leading up to and within the sponges that are easily ruptured by torquing or pulling on the penis. You can gain a little length from months of continued "exercising", but you could far more easily damage your penis by breaking these blood vessels or even permanently rupturing the spongy tissue, resulting in the dreaded 'broke ****', as colloquialized by Hollywood.
There are only two ways to affect your penis size: one is surgical, the other is supplemental. The surgical procedure that almost all **** stars go through to gain instant inches is to sever the suspensory ligament. The caveat of this procedure is that you get a 'dead' penis, meaning you are no longer capable of moving it or holding it up by flexing this ligament, which is why **** stars so often have to hold their penises up.
The supplemental answer is tried and true but MUST be prolonged and will reverse completely with stoppage of taking it. It's called L-Arginine, and it's a simple supplement sold at any GNC or similar store. Its a natural substance that promotes stronger bloodflow in extremities--including your penis, and it is the active ingredient in Enzyte. Most users report greater length, girth or both, but all who take it also get a more sensitive penis as well as sometimes fingers and toes, and many report a 'tingly' sensation that is not unpleasant.
Consider carefully though, that your penis is well within average range, and that most of a woman's pleasure zones are within the first 3" of the vagina or are external; your length means little in regards to this.