How can I modify the state machine logic?

typedef enum {
  STATE_IDLE,
  STATE_COIN_INSERTED,
  STATE_SELECTION_MADE,
  STATE_DISPENSING
} VendingMachineState;

typedef struct {
  VendingMachineState currentState;
} VendingMachine;

extern VendingMachineState GetCurrentState(VendingMachine *machine);
extern void SetNextState(VendingMachine *machine, VendingMachineState nextState);