Hi thanhnambka,
OTP is accessed with API. You can refer otp demo code under MCUXpresso SDK to know how to use OTP API.
Function OTP_ProgramRegister, which calls otpProgramReg API, is to set OTP register. BOOT_SRC is at OTP bank 3, word 0, bit 10:9.
For more information about this register, see UM11060 45.12.4.3 OTP memory bank 3, word 0 - Boot ROM control data.
Please note, OTP is One-Time Programmable memory. That's to say, once OPT is written, it can't be written again. No matter you do system reset or not. Please double check your setting and make sure it is correct before programming OTP.
Hope this helps,
Jun Zhang