If you are using supermarket apple juice, I'd just buy the frozen cans of concentrate. If you are using fresh pressed cider, you can concentrate it by freezing. You can also concentrate by heating, but that changes the flavor.
I have freeze concentrated unfermented cider quite a bit. I used to partially freeze the cider and then skim off the ice. Now, I let the cider freeze solid. I've found this to be more efficient with a fraction of the labor needed for the partial freezing method. The most concentrated cider comes out at the beginning and gets gradually weaker until you have mostly water left. I divide it roughly into thirds. The first third is usually high gravity. The second third gets refrozen, since it's a bit weak. The last third gets dumped, since it is mostly water.
For small batches, you can freeze the cider in gallon plastic jugs. Let them freeze solid. Then, invert them and let them drain as they melt. If you are careful, you can balance them on a pint glass to drain. Empty the glass as it fills.
I prefer to freeze cider in buckets. I use a traditional fridge with freezer on top. Five gallon buckets and my taller bottling bucket will fit in the freezer. A bucket with a spigot is best. After freezing, just set it up so the spigot drains into a gallon jug or carboy. I've also used buckets without spigots. After freezing, I let the bucket melt a little, so the frozen cider isn't stuck to the sides and bottom of the bucket. Then, I balance the bucket of cider at a 45 degree angle on top of an empty bucket to drain. It sounds odd, but it is quite stable. Balancing the inverted bucket without melting a little first can cause problems. You don't want to balance the inverted bucket with the cider stuck to the bottom of the bucket.
The bucket on the left has been draining longer. You can see that the ice is almost white.