How do you edit an existing texture pack?

[Dr. Zuko]

Hello there, so I have been using a texture pack that I like a lot but some of the textures to certain machines are not textured. From what I have heard, the creator of the texture pack does not do mod support :/ So I decided to start creating those textures in his own style...the problem is that I do not know how to actually edit a texture pack. Could anybody give me any instructions on how to do so? Also, these are textures for Tekkit Lite specifically.