How do edit someone elses Team Change GUI?

Use ROBLOX studio.

Start studio. Insert the GUI you want to edit, find it in workspace. Then if you don't know much on the properties of GUIs, you can easily look it up and it's very easy to understand. After that, place your edited GUI into StarterGui and save.

Put the gui in StarterGUI first.

If a free model or unedited,

Then you want to change the name of your clan. Go to the Text of the defending clan and click it. Its in the Parent/Child thing (forgot the name for it). Once clicked, go to properties, find "Text" (its good to change the name too.) and put in your clans name.

That's done.

To make it work, find the childScript in the text. Open it up. Find where you put the group ID. Copy/Paste your clan's group ID into the spot where the other group ID is in. Done.

Also, check the scripts of the other buttons. Change the colors stated in the script to the actual team color you have set for it.

And done!

If this wasn't explained good enough, PM me and I could help you in more detail.