You can install Xfce with sudo apt install xfce4 xfce4-goodies or you can install the base Xubuntu by: sudo apt install xubuntu-core^
If you so choose and are using Ubuntu MATE and the MATE desktop you can remove it afterwards by: sudo apt remove mate-desktop
I would recommend performing a backup prior to doing this. I would also recommend making sure your new DE is working before removing your current DE.