Hi! I Hope I'm asking this in the right place!...

I will try to be as respectful as possible because the creator of this world did an amazing job making the world I have a questions about, so If possible, I won't reveal the world in case it would come off disrespectful.

There's a custom world that I love, but the grass and the roads and the sand in the world I really would like to change since it is not to my liking.
I know you can use s3pe, but I can't find steps on how to change the terrain for the world to the textures I have. I downloaded the CAW tool, but it kept saying there was an error and wouldn't allow me to open it.

Is there a way to change a custom world's terrain? Also, I don't plan on reuploading the world if I happen to change anything, not only is it disrespectful to the creator, but I'm only using this for my own personal use.

There are two ways to achieve this.

1) using S3PE and replacing the image ressources for the respective textures. You have to extract the world from the sims3pack with Delphy's Multi-Installer. Rename the resulting package file to whateverthenameoftheworldis.world. Open with S3PE and then you will have to find the ressources. They will have the IMG tag so you can sort by tag. There are quite a lot IMG ressources so it might be some needle in a haystack work. If you found them, export to file (this will be a .dds file). This file you have to open in an image editor that can process .dds-files. Gimp can, paint.net can, also Photoshop with a plugin. Open the file, put your desired texture on top, remove the original texture and save. Open the world again, find the original IMG ressource and right-click "replace" texture. Save.
This you will have to do for each texture.
After exporting and changing the original textures, you could also add them to a new package file with S3PE and put that package file in overrides, making it easier to change/remove/restore.

2) Edit the world in CAW. There is this thread: http://www.modthesims.info/showthread.php?t=469942 with a detailed process. Basically, you extract the world from sims3pack to a package file. Open the package file with s3pe. Export ALL ressources to file. Make a new world in CAW (largest size and height, flat). Save that world in CAW and close everything. Now open the CAW .world file in your CAW user folder with s3pe, import all those files. Save (just save! Not save as!). Close everything. Now, when you open the world in CAW again, you can edit all textures to your liking.

Both methods have the disadvantage that the original textures will shine through under lots in map view. You would have to renew all lots in CAW to avoid this as sadly the terrain information is not updated if you change it under a lot, it is stored the moment you add that lot in CAW.

Thought I'd add that you can circumvent most visual issues caused by replacing world textures by using Shimrod's Camera Mod, or the BrntWaffles edit of it, because it replaces the Map View camera with a proper zoom out state rather than the pseudo-LOD of the world that the game uses by default- which is the part that has those old textures painted onto it.
That's how I'm able to play with the Midnight Hollow terrain retexture and not see any issues ingame

Thank you both so much! This is very helpful!!! ^_^