I have not seen the film but it looks like the procedure of plastic surgery to me and even wikipedia description say that.

Houstonpress elabprate it little better:

Sook-hee is then captured by a secret wing of the Korean Intelligence Agency and compelled to become a government assassin, in exchange for eventually getting her life back. With a new name, a new face (thanks to some amazing plastic surgery) and a young daughter, she’s released into the world, whereupon she promptly meets handsome widower-next-door Hyun-soo (Bang Sung-jun), who secretly works for the same agency and has been ordered to keep tabs on her.

But she was played by same actress before and after surgery raising soem confusion but her looks did chnage to an extent with styling.

I have not seen the film but it looks like the procedure of plastic surgery to me and even wikipedia description say that.

Houstonpress elabprate it little better:

Sook-hee is then captured by a secret wing of the Korean Intelligence Agency and compelled to become a government assassin, in exchange for eventually getting her life back. With a new name, a new face (thanks to some amazing plastic surgery) and a young daughter, she’s released into the world, whereupon she promptly meets handsome widower-next-door Hyun-soo (Bang Sung-jun), who secretly works for the same agency and has been ordered to keep tabs on her.

But she was played by same actress before and after surgery raising some confusion but her looks did change to an extent with styling.