How to increase average speed?

My bmi is 25.I ride on a mach city I bike which costed me around 6k and it doesn't have gears nor disc brakes. So I can't go beyond certain pedalling , also have to plan about breaking way before. When I went near 23-22 bmi, progressively I got worse at cycling. I wasn't able to keep up. It's like my weight was assisting me before. Also since my cycle has steel rims, I don't get the flying feel. My avg speed was around 21 km/hr which reduced to around 19.5 km/hr. How to improve?