Which one do you have? The short answer is "carefully" but you may need to gently pry one end with a micro-screwdriver... preferably a flat head type. Gentle is the key, but the LL locos I have are a VERY tight fit.
Every new Life-Like locomotives has an instruction sheet hidden in the bottom of the plastic box.
That sheet has an exploded view of the locomotive and ... the instructions about how to take the model apart.
I can tell you how to take apart a SW9 or a GP18.
On a GP38, there are four slots on the bottom of the shell. I put a flat-bladed screwdriver on one side, then go to the other side and gently pry with another screwdriver starting on one end then the other.
On a GP20, the center vent is removable, I took that off and was able to push down on the frame. The same appears to be the case with a GP18.