You can buy all sorts of hedge cutting tools. some on extensible poles. We have a range of Wolf tools that hubby uses to keep the 16ft beech hedge in check. There are a range of other bits and bobs. such as shears up to chain saws.
if the hedge is very tall it may be worth paying a professional to bring it down to an appropriate height first, as ladders are unsafe really. We did that originally and the major bonus was they took all the stuff away with them. We now give it a trim twice a year.
hope this helps.

I think that If you cut back into old wood they do not usually regrow. If it's a big job then perhaps it needs someone with a chainsaw who is qualified and insured to tackle it.