For questions like this, you can find the answer by using collocations and grammar.
So you know the original sentence is a threat, so ask yourself which piece or grammar we use for threats (and this phrase uses the word "better"). That will also tell you the form of the verb.
The preposition "in" will tell you another missing word. Study phrasal verbs and dependent prepositions. "Let me have your homework" is usually said by a teacher, and we know that we "submit" essays, assignments and schoolwork. So is there a phrase that means the same thing, and uses "in"?
I realise shevelen8 has already given the answer, but I hope my advice will help you with future transformation questions.