Translation:How cold it is today!
I think "Today it is so cold!" is a reasonable alternative; one can read "today" as being either the subject (as in the official answer) or a time in a Chinese sentence missing a subject (where "it" would need to be used).
"It is so cold today!" is accepted; while it is acceptable to put "Today" at the start, this is more idiomatic.
Because it is not a question but an exclamation (How cold it is today!! It is so cold today!!). Your sentence is a question and would translate as 今天有多冷？
But in English its a rhetoric question. Your stating a question and not expecting a reply
"Today it is so cold!"is a better translation ,what sounds more like correct english.