How to make a variable change the text in a GUI

The title gives it all, I want to be able to show the players certain class exp in the menu but don't have any clue on how to do it. I found a few posts talking about text fields but they never went more in depth than that. Could someone give me an idea on what to do here? It doesn't have to be words either, just a changing number value since I need to be able to alert the player of how much exp they have when they upgrade the skill tree.