This command will change the appearance of your targeted NPC (or character if no target) to the appearance with the specified ID. The appearance ID should have apostrophes on either side: appearance('ciri_winter'), not appearance(ciri_winter).
The syntax for the appearance command is as follows:
This command has the following arguments:
|'Appearance ID'||The ID of the appearance you wish to change your character's to, with apostrophes on either side of the ID.|
Find below working examples of the appearance command.
The above command would switch your appearance, or (if you have a target selected) your target's appearance, to Ciri's winter model (Ciri in a coat and winter clothes).