The simplest way is to increase weight and lower the reps. You have to overload muscles in order to get them to grow.
How tall are you?
My husband is 5'10 and 153. What he's got is mostly muscle. I think he needs to eat more if he wants to bulk up his arms a little more. Any tips from all you lifters? He wants a bigger chest and a bit bigger arms. He's interested in a lean muscle look not real big.
Also the nutrition advice other people gave is good info.