Update first your system to receive the latest updates
This will force a refresh of all package lists and upgrade all Arch and ArcoLinux packages
Then you check if you need to update the packages coming from AUR
Now it is up to you to install the desktop of your choice.
Choose from the phase2 menu to choose xfce, openbox or i3 and install it.
Choose from the phase 3 menu to choose one of the other desktops and install it.