Gfilui

I have had Budgie installed since 16.10, and now I'd like to remove it. The instructions in the answer to Completely remove Budgie are for a system with several desktops installed. The answer to How to uninstall Budgie Desktop is for a version of Ubuntu before it came with Gnome. I just updated to 18.04.

I'm a simple user. I can follow the logic of the instructions of the 2nd linked question above on how to get to the Grub menu and open a terminal there, and I already looked at an article that explained the first few lines of the code that follows that - mounting the drives so they can be written to, updating.

I get lost at apt-get install --reinstall aptitude deborphan ppa-purge.

Should I follow those previous instructions to remove Budgie and switch to Gnome, or is it different now?

Try this:
Start in text-only mode

Switch on your computer. Wait until the BIOS has finished loading, and press and hold Shift, which will bring up the Grub menu.
Select the line which starts with Advanced options.
Select the line ending with (recovTry this:

Start in text-only mode

Switch on your computer. Wait until the BIOS has finished loading, and press and hold Shift, which will bring up the Grub menu.
Select the line which starts with Advanced options.
Select the line ending with (recovery mode)
Press Return and your machine will begin the boot process.
After a few moments, your PC should display a menu with a number of options, including Drop to root shell prompt. Press Return with this option highlighted.
The PC will start in a terminal.

Run these commands:

Mount partitions in read-write mode

mount -o remount,rw /
mount --all

Edited on 20-05-2018

Start networking:

systemctl enable NetworkManager.service
systemctl start NetworkManager.service

Update repositories

apt update

Install aptitude and deborphan

apt install --reinstall aptitude deborphan

Eliminate the components of budgie that are not necessary in Ubuntu

aptitude remove '?and(?reverse-depends(budgie),?not(?reverse-depends(?exact-name(ubuntu-desktop))))'

Reinstall ubuntu-desktop

apt install --reinstall ubuntu-desktop

Eliminate orphan packages

deborphan
apt --purge remove $(deborphan)
deborphan --libdevel
apt --purge remove $(deborphan --libdevel)
deborphan --find-config
dpkg --purge $(deborphan --find-config)

Remove unnecessary packages

apt autoremove

Remove downloaded packages

apt clean

Restart system

reboot

When restarting, remove the budgie ppa.

sudo ppa-purge ppa:budgie-remix/ppa
or
sudo ppa-purge ppa:ubuntubudgie/backports