You have a number of options.
1. Bloated option (my favorite)
Add the Lubuntu desktop to your existing system.
sudo apt install lubuntu-desktop
It'll add LXDE & all Lubuntu software to your existing system. It increases bandwidth for each update (as you'll get all Ubuntu (GNOME) + Lubuntu (LXDE) upgrades; it also complicates menus somewhat (eg. you'll have a load more programs being for example Ubuntu GNOME's gedit
text editor, plus Lubuntu's text editor leafpad
... It's probably not ideal for a 'newbie', but it lets you use LXDE (Lubuntu) when you want to, or at login you can select to use GNOME (Ubuntu).
2. Re-install Lubuntu 18.04 LTS
Download Lubuntu 18.04 LTS ISO, write to install media & install.
Use the 'Something-else' option, select your existing partitions ensuring you don't have format (esp. for whatever partition has your /home partition). It'll cause the following to occur
- any added packages will be noted
- system directories will be erased
- new system installed
- additional packages you had installed (noted earlier) will be added back
- you're asked to reboot.
Just ensure you don't format your home partition (it'll be in "/" if you didn't specify another partition), otherwise what I've described here won't apply.
3. Switch to Lubuntu 19.10 during install
I'd be tempted to use this one if you're sticking to Lubuntu. Lubuntu 18.04 LTS was the last that used the LXDE desktop, as all later releases use the more modern LXQt. Yes it's possible to upgrade without re-install (I did on this box), however due to problems doing it, it's not supported & not recommended. Thus you could make the switch now, and be running a system that'll easily upgrade to Lubuntu 20.04 LTS in a few months.
Download the Lubuntu 19.10 ISO, write to media, and install using "Manual Partitioning" (what was called Something-else in 18.04). Select your existing partitions ensuring you don't have the 'format' boxes checked, it'll do the same as the prior install.
- any added packages will be noted
- system directories will be erased
- new system installed
- additional packages you had installed (noted earlier) will be added back
- you're asked to reboot.
I'll add some links
The Lubuntu manual (Installation section)
https://manual.lubuntu.me/stable/1/Installing_lubuntu.html
Chapter 1.1 Retrieving the image - https://manual.lubuntu.me/stable/1/1.1/retrieving_the_image.html
Chapter 1.2 Booting the Image - https://manual.lubuntu.me/stable/1/1.2/booting_the_image.html
Chapter 1.3 Installation - https://manual.lubuntu.me/stable/1/1.3/installation.html
The manual pages refer to 19.10; which is treated as the stable release. As the manual was started with the switch to LXQt, it doesn't really relate to LXDE or 'legacy' Lubuntu.
You didn't provide specs of your box; my own box is a decade old & I'm running LXQt correctly (20.04), however for x86 (i386 or 32-bit boxes) the LXQt is not a choice and 18.04 LTS is your best bet. If you're running a x86_64 (amd64) box, I'd really consider using this as a chance to switch to 19.10 and LXQt. It'll be the easiest longer-term. Lubuntu 18.04 LTS is supported until 2021-April (ie. 3 years from release in 2018-April).
sudo apt install lubuntu-desktop
, then logout & try Lubuntu's LXDE interface on your existing system. If you're happy, you can just keep that (my install is a Ubuntu one, with Lubuntu, Xubuntu... added to it There are costs (the more you add), but it'd be my option. I can expand is helpful. – guiverc Feb 22 '20 at 07:42