1- As for هو واحد it means whether you change the way it’s written or not, and whether you distort it or not, Allah will protect it, so don’t think you can change the Qur’an. “It’s the same” whether you distort it or not—Allah won’t allow you to do that. He’ll expose it and you won’t get away with it.
2- Another hadith explains that what is meant by هو واحد is that Jibraeel will fix what you change. So after you write it incorrectly, Jibraeel will edit it in your copy and fix it, so you won’t be able to change the Qur’an.
3- As for دعها it basically means: you want to change it عليم حكيم from عزيز حكيم? Well, Allah is aware and He is wise, and so he will protect His word and fix it. You want to distort it? Do what you want, Allah the knowing the wise will basically fix it. This is what it means it. This is one understanding of دعها
The second understanding, which is more plausible, is that the Prophet (s) told him: no, keep it عزيز حكيم not عليم حكيم even though Allah is علم حكيم. So don’t change what I tell you. Keep it as is.