User blog:Sylphoid/BTD6 Data and Asset Extraction

Extractor Selection
UABE cannot open bundle file types, nor can it open encrypted asset file types either, so an alternative extractor is needed. For this case, AssetStudio is a good alternative as it bypasses the password requirement and allows you to preview the textures and sprites from directly inside the app. To install, click the badge and select the latest version that corresponds with your computer processor. After installing, extract the zip file, and run the exe. If it does not run because of .NET errors, you will need to download .NET Desktop Runtime.

This video tutorial also has pretty good steps to follow https://www.youtube.com/watch?v=zgglKNFbvfk.