How do I change my username font to something like "Lucida Handwriting" instead of the boring block font?
Here I assume by username you mean profile name. If so, you must know officially Facebook doesn't allow to change the font of the name. See What names are allowed on Facebook?.
Some people use different font/style using some script or third party tool (like Unicode Text Converter). You can also do the same. Change the font of your name and copy paste it on profile. But this have some disadvantage like your name will not come when someone search for you because the characters you have used won't be Facebook data base.