You should read on chapter 3 (non secure) and 4 (secure) based on your specific need.
Based on ISP / OTP bits, NXP boot allows you to download and launch a firmware from USB into RAM. If you want to program internal flash then you need to write a firmware to do or you can re-use mcu-boot (part of the SDK) to do so. i.e. :
1) ISP download MCU-boot from USB to RAM
2)use blhost.exe to interact with MCU-boot to actually program your custom firmware into flash.