Since NaNoWriMo may well have taken up much of your time for a month, it makes sense to have a break. Whether you want to lengthen your novel or, for young adult publishing, tighten it up, January is the perfect time to do it. For one thing, you may feel inspired by the New Year's Resolution atmosphere. For another, one month is the perfect amount of time to rest a novel.
Editing
Hopefully you will now have enough distance from your piece to assess if a). it is any good and b) how much is worth keeping. Maybe much of your dialogue is strong or the basic plot is great, but the ending was a little rushed. It may be an idea to start a new file to put in all the bits you like. Don't be discouraged if this new file is smaller than the original. You will know these sections are actually solid. You may want to ask for a second opinion from a friendly writing group if you are unsure.
Lengthening
Unless you are writing a children's or young adult book, chances are the piece will need to be considerably longer. You can either go back and weave new elements into your NaNo piece or decide to carry on writing from where you left off. Neither approach is necessarily right or wrong; it depends how you work. Either way, you should have a strong position to start from.
Scrap It
This is the most extreme option. Perhaps the piece you were writing just doesn't work. Don't be upset. This is a plot bunny you've got out of your head and there must be at least some elements you can still use.
Remember that NaNoWriMo is a starting place. Whatever you decide to do, you have a lot of work still to go. Good luck!
