Due to the different molds, there are two installation and disassembly ways depends on type of your doll, one is rotary installation and the other is snap-in. Your doll may get one of them randomly.
For rotary installation, which can be installed and disassembled by rotating the feet;
For snap-on installation, which is easy to install to leg just use force to push in. for disassembling, best use a tool like hanging hook as the video below to push the round plate in the buckle on the thigh, at the same time, shake the leg to pull it out. refer this video to know the way how to install and uninstall:
snap-on installation & removal with a tool
snap-on installation & removal video without a tool.