iFixit has torn apart the Galaxy Note 9, to see how easy it is to repair. This is typical of iFixit, as they tear down just about every major smartphone, tablet and a few other electronics as well. The Galaxy Note 9 got a repairability score of four out of 10, with 10 being the easiest to repair. It's the same score that the Galaxy Note 8 got last year from iFixit. That isn't a surprise at all, given the fact that the Galaxy Note 9 is virtually the same, from a build and design perspective, as the Galaxy Note 8.
In the tear down, iFixit notes that the Galaxy Note 9 components are more modular than ever, partly due to the USB-C connector in the Galaxy Note 9. This is going to make it easier to repair some things, but not everything. iFixit also notes that the Galaxy Note 9 only uses philips screws here, and nothing else. The battery is replaceable, if you are determined, as there is plenty of glue on the Galaxy Note 9 keeping everything in tact. But the battery can at least be replaced. But, getting inside the Galaxy Note 9 is going to be tough. As you are going to need to unglue the back panel on the Galaxy Note 9 - and then re-glue it when you put it back together. Which isn't going to be easy.
The Galaxy Note 9 is a really good looking smartphone from Samsung, and it's going to be pretty tough to repair yourself in DIY repairs. Luckily, Samsung has partnered with uBreakiFix for repairs, which offers same day repairs for many repairs on the Galaxy Note 9. So you can go into a local uBreakiFix store and get your Galaxy Note 9 repaired if you want. Remember that taking apart your Galaxy Note 9 does void the warranty, so it's a good idea to avoid doing that if possible.