It's just a push fitting, it will pull apart (right below where the button sits). Just firmly pull while wiggling it and it should come apart (you can use needlenose pliers or something if need be but I never had to with the
smok ones).
Just two things to consider when doing so:
1) Once it comes loose and starts coming apart be very gentle in how you pull it the rest of the way out. If you're too rough you can accidentally pull a soldered wire out of something, and your
device will be broken unless you can solder the wire back on.
2) Once you pull that push fitting out once it will be semi-loose and will pull out much easier than it did the first time, even accidentally in your pocket or whatever.