Learn Swedish: Annars kommer du att frysa om huvudet. - Otherwise your head will be cold.

When it is obvious from the context whose body part is involved, Swedish usually uses the definite form (like huvudet) rather than a possessive pronoun (like ditt). Since you are the one feeling cold, it is naturally your own head.

Why use the preposition om here?

To express that a specific body part is cold, Swedish uses the phrase att frysa om (literally 'to freeze around'). So instead of saying 'your head is cold', Swedish phrases it as 'you are freezing around the head'.

Why is the word order kommer du instead of du kommer?

This is the V2 (verb-second) rule. Because the sentence starts with the adverb Annars (Otherwise), the first verb kommer must be the second piece of information in the sentence. This pushes the subject du to the third position.

Why does it use frysa for 'be cold'? Couldn't I just use vara kall?

When referring to a person feeling the sensation of cold, you always use the verb frysa. The adjective kall is mainly used for objects, weather, or to describe someone whose skin is physically cold to the touch.