✓ Accepted answer
The default leaderboard GUI element that is generated by CoreScripts using 'leaderstats' is not customizable.
What you could do however, (and what the majority of well-made games are doing now) is create your own leaderboard GUI. Simply design the GUI in the fashion you wish and populate it with the information you'd like to use (you can use a structure similar to 'leaderstats' to store data, or do it however you'd like.)
There aren't any wiki articles on the subject, but it's pretty self-explanatory. It's a little more involved than simply using leaderstats, but will ultimately give you much more control over the exact look and function of your leaderboard.
2 comments
That isnt well documented. You can always look at the corescripts though. In particular Topbar.lua, line 759 should be what you're looking for. https://github.com/Roblox/Core-Scripts/blob/master/CoreScriptsRoot/CoreScripts/Topbar.lua
U simply cant.