As you can see, above this post is my username. You might want to change your username because of privacy reasons, or just for fun.
1. From the home page, go to your profile picture near the top right and click to get the drop down.
2. Click on either "Profile" or "My Account". Doesn't matter which one you pick.
3. On the left there will be a box with your profile picture and it will show your username which will most likely be your full name.
4. Click on the purple box that says "Edit" and edit your username to whatever you want!
I hope you found this useful!