Sure, it's simple (this is also in the new Help file in the new Editor)
Open the editor, go to the Objects tab. The left scroll list is all of the available shapes. Scroll down and highlight (for example) the F6F5. Now click on the Save Texture button in the top right. The editor has just created a folder named "Savedtex" in the Terrains folder in your Editor directory. Inside here are all the BMPs that make up the F6F5... wings, gauges, everything. Find the one you want to edit, open it in the bitmap editor of your choice (PhotoShop rules).
For some reason the editor exports the tiles inverted... meaning they are flipped vertically (not sure if this is a feature or flaw, but it is easily fixed) you will want to flip it before you're finished.
Do your stuff to the BMP (make a copy and save it somewhere first) , make sure it is 8-bit (indexed color) and put the edited texture into a folder called "Texsrc" that resides in the folder of the current terrain that you are working on.
Open the terrain editor. You need not do anything else but Build at this point. If you haven't actually made your own terrain yet, maybe you should slow down and learn to do that first.
Now try your terrain offline.
It's all pretty simple. Make sure you read the Help file in the Editor in case I've missed anything. Good luck!