What is userstats.lua? This file stores the players data that played on the server.
What kind of data? Well that is the thing that shows up when they say rank.
Can I edit it? Legally of course you can, but it will be quite hard to do that so you will need to use a program designed for that in order to do that.
What program do I use? You can try finding a program on unrealsoftware or ask someone who knows.