You won't kill an apple tree by pruning it and December is a good month to do it. I pruned the trees in my young orchard today.
It's beyond the scope of a short commentary to tell you all about it, but assuming it is relatively immature tree (less than 5 years old), you could safely prune the leading shoots by at least 50% of the growth they put on this year. Prune back to a bud that faces inwards, if the natural habit of the tree is grow outwards and you want to encourage more height.
The objective is to pursuade the tree to grow a framework of branches in a broadly even shape.
There's a bit more to it than that, but that should get you started. You will probably constrain the quantity of apples next year, but it has to be done to avoid a silly shaped tree later.