how do i do that?
Instead of replacing textures in the media folder, place new files and give them names not already used in there.
I can't remember how to extract the XML of an UBOX file, but if you search on the forum, I'm sure I've read it been written before.
In there, just edit the XML tags so that they refer to your custom texture instead of an existing one.
what do i have to edit
The text inside the XML tags refering to the texture of a body. It has been explained on this forum before, so I recommend searching the forum.
and how will i be able to open in it via simulator.
You could right click the XML file and select 'Open with...', then choose Universe Sandbox. You could also place in your systems folder and select it from the program.
Sorry if any of my information is wrong. I haven't tested or checked it, this is just how I remember it. I don't have time to test/check it right now, so for a better explanation, search the forum.