Download the .zip from the nightly link for the OS you're using. you can complie from source if you went but I'm lazy and trust the software enough, so I just download from the nightly.
Open UABEANext and open folders by using files -> open folder or ctrl + shift + o
Locate your OPERATOR folder: right-click OPERATOR, and then manage -> Browse local files.
All the files should be in OPERATOR_Data -> StreamingAssets-> aa -> StandaloneWindows64. (it will probably take sometime to load)
Find the .bundle/assset where the texture you want to replace is at(red)
Now find the file(s) you want to replace (red) you can use the filter to find what you need if theres a lot
You can use the "preview" button (red) to see if you've got the right one
Now once you've got it, click plugin (red +you don't need any "plugins" that you need to download, that's just what it's called)
Click export and edit it
lastly click plugins and import, and select the folder where your edited texture is located, and this dialog box will popup and show a list of the 2DTextures, find the one you edited and click OK
To now save it to actually use it, go to files -> Save Selected As or ctrl + shift + s