Wait until spring - if you prune now you will trigger fresh growth and it's more susceptible to freezing.

Yellow leaves? Even "evergreen" plants will shed leaves, but if you have a lot of them that are pale greenish, it might need some fertilizer.

Also, how much sun does it get? They like east-facing exposure.

I think your hibiscus plant looks pretty darn good. If it was mine I wouldn't really do anything with it at all. Like lazy mentioned it's normal for them (and citrus and others) to shed their oldest leaves now and then. Yours looks good to me. If you feel it needs to be pruned, I agree with lazy again--wait until early to mid-spring. That way if anything gets damaged by cold you can trim that off too and because trimming encourages fresh young delicate growth and you don't want that right before winter in case it's a frosty one. If your plant was mine I wouldn't change it at all, other than making sure to give it general all purpose fertilizer every few weeks in spring/summer/early autumn (again not in late autumn since you don't' want lots of tender new growth before winter). Let us know what you do and how it works out! Happy gardening!