If you are using the old LPCXpresso IDE, then see : Relocating code from FLASH to RAM .
If you are using the newer MCUXpresso IDE, then there is an updated version of this information in the supplied MCUXpresso IDE v10.0.2 User Guide, chapter 12 - in particular section 12.11.1,"Relocating code from FLASH to RAM".
If you are not using either of these toolchains, then I suggest that you confirm which toolchain you are using, so that someone can give you appropriate advice.
Regards,
MCUXpresso IDE Support