You must first pry the spring-loaded ring. If you have a needle or a very small screwdriver, just insert it between this ring and circuit board and try to prise the ring gently. If you do it on each side by a small steps, it'll be easier. If you remove the ring, just unscrew the circuit board with the needle-nose pliers or some forceps. The light engine is not glued. Good luck !
EDIT: Judging from the photo, you have some different construction ... My NDI has anodized threads on both sides and a spring-loaded ring in LE for switching between high and 'user' mode. It seems you have some simplier construction. If you cannot unscrew it with needle-nose pliers or forceps it means that it's glued and without heating disassembling may be hard.