Dude, that's not below average. What is below average is the results of penile enhancement surgery, which is the only way you're going to add a for-real inch to either dimension, so....
Also, you're talking an inch, dude. What difference is that going to make? Is a woman really going to notice the difference between 7x5 and 8x6? Babies come out of those things. No, I'm guessing it's for your self-esteem, in which case you really should work on that and learn to love your dick. Good luck, man.