How to edit UI?

Hi, I was just wondering if and how I could edit UI. I suppose i should say what im wanting to do first: I want to download the analog gauge pack, but I want to change some of the looks of it such as the needles/faces ect. I dont plan on messing with any of the coding of it, just the looks of it. Is there a program i need for this? Or is it as simple as loading it into something like Microsoft paint. Thanks!

 

You can edit the Analog Gauge pack by just changing the images in it. Changing the angles (where 0 and max is) is the complicated part, if you have to do so though.

 

Do I have to size the images correctly or anything like that? Or simply find a picture of a gauge needle (for example) and replace the old one

 

I forget whether or not they have to be the same size. Most likely not though