Any way to change Roblox Guis?

Although you cannot change inbuilt things like that, you can remove them and create your own. They are both extremely simple to make!

You cant change how they look, but you can disable them and make your own! but it is kinda hard, id say you should have intermediate knowledge of what your trying to change(health bar, leaderboards, etc.)

this is the link that does what you want:

http://wiki.roblox.com/index.php/SetCoreGuiEnabled_(Method)

the code will say this:

SetCoreGuiEnabled("CoreGuiType" coreGuiType = All, bool enabled = false )

If you click CoreGuiType(the one in quotations), it will give you a list of stuff you can disable

So lets say you want to disable the leaderboard and make your own, you would do this:

SetCoreGuiEnabled(coreGuiType = PlayerList,  enabled = false )

the line above will disable the Roblox leaderboard that roblox provides, but you will have to make your own leaderboard. hope this helps :)