The easiest way is to wait for the installation/update to complete, then using your gnome terminal enter:
sudo apt-get install xfce4
this will install xfce4 desktop.
Once that’s done, log out of the system and head to the login screen
Once you’re there, select your username
Then before entering the password, look to top-right or bottom-right for an option that lets you select the desktop environment (GNOME, XFCE, all the ones that you have installed).
Select XFCE then enter your password to enter XFCE desktop
Once you’re there, run terminal.
Then in terminal, enter sudo apt remove gnome-desktop
(or whatever is GNOME package’s name is, I forgot tbh)
Then once it’s uninstalled, enter:
sudo apt autoremove
sudo apt autoclean
That should remove gnome-desktop packages and the environment from your system. After this log out again to head back to the login screen.
Once you’re there, enter your username and press enter, but don’t enter your password.
Find that option once again that lets you select desktop environments.
If you find just XFCE over there, or if you’re unable to find that option this time, both of these mean you have successfully deleted GNOME and now XFCE is your only DE now. enter your password now and login to xfce.