Okay, I'm sorry in advanced if this is a stupid question but:

I want to edit this premade world and I'm in CAW and I don't know how to remove these huge obnoxious hills.
There's a section in the world with a large hill, with roads and lots circling it, and I don't want it in my game. Instead, I just want to completely remove that section and place water there.

So, I guess what I'm asking is if there's a way to lower hills in CAW? I can't find a way how.

The easiest way, I think, would be to flatten the area first by pulling the flatten terrain brush at 100% strength from an area that is level with the main land mass and then use the valley or steep valley brush to "push" or "dig" the land down into the water. I believe you could eventually lower the hills with the valley brush but it will take a lot more effort. I hope this makes sense. If not, please feel free to ask for a clarification. Of course, you will want to remove the roads, lots, trees, or any other objects before flattening the area to avoid possibly burying objects. Good luck with your project!

Another thing you could do is export the height map. Then open the .png file to a photography editor and color the area you want to remove black. Then save and import your new .png file. If you want to lower the hills height but keep the same size you could also color over the hills with a darker grey. With this white means it's as high as your elevation will allow, black is sea level. Save and re-import your height map.
Let me know if this is something you'd like to try and you'd prefer pictures I could provide them. I use Photoshop, but let me know what program you use and I'll help, I have yet to use Gimp or Paint.Net but I can take a look at those and learn them as well to help you.

SimmyRN's suggestion worked for me (thanks, by the way!), but it took me hours. This seems a lot simpler and I'd like to learn how to do it for future tweaks I'd want to do, but I have no idea what height maps are. Yeah, I'm really, really inexperienced with stuff like this. But I have Photoshop too, so... I'd like pictures, yes.