Pry open the rounded edge that is part of the main steel tube (right behind the O-ring) until you can pull out the head (item on the bottom left of the image). The center item shows the pried open logger tube with the head removed.

Alternatively you can grind off the bend edge. The logger will still be water tight upon re-assembly as there are 1-2 O-rings on the inside and grinding away the edge will not interfere with the O-rings. However it won't be easy to secure the head in place without the metal edge.

Do not twist the cap when pulling it out as it could break off the IR diodes in the process