28 km/h is the maximum speed with the regular battery. To go faster you need to overvolt (add additional batteries).
1. Download RollerPlausch PRO - Firmware (1.5.5) PRO - FULLSPEED or [url=http://сяокат.рф/ru/konstruktor-proshivok-pro]generate[/url] a firmware with "Maximum speed in Sport mode" set to 30+.
2. Use m365 DownG to flash it.