How to gain weight for upcoming competition?

i'm 148 pounds right now. i train in Muay Thai, kickboxing, and Jiu Jitsu, and i plan on competing again in kickboxing in december. last time i competed i was in the 145 - 160 bracket, and this time i would like to move up to 170. how would i be able to gain this weight without some crazy diet. i'm 15.