I have Ubuntu and want to replace it with xubuntu, is there a

open up software center and install xfce
Restart and pick XFCE when you log in

>>16410
No. Xubuntu is not just Ubuntu with one flag changed. You need to add the Xubuntu repositories and replace the packages that are different. It's close enough to Ubuntu that the dependencies of Ubuntu's base virtual packages should do this for you.
>>16394
Replace your sources.list with xubuntu's sources.list*, then apt-get update, apt-get upgrade.

* which appears to be super-secret classified information.

>>16446
this.
Else you can simply install xubuntu on top of ubuntu without removing partitions or anything if you want to do a clean install

sudo apt-get install xubuntu-desktop

there that was hard, also ignore ^ those fucktards