Using Toolbox (original) (raw)
First download the latest build here
Right click the zip and go to properties.
Check the "Unblock" option

Then click "Apply" and "OK"
Lastly, you need these 2 required installs
These are required for most texture viewing as it is the main library to decompress/compress these back.
Now open Toolbox.exe.

You can now use the tool. File - Open to open new files. Files can also be dragged/dropped to the tool. The green arrow is for automatic updates.