Fix a few problem in Ubuntu/Xubuntu 18.04 Bionic LTS Software Center, Steam Games, Virtual Box VM. 

To start ALWAYS HAVE A BACK-UP OF EVERYTHING YOU EDIT, CHANGE OR ADD.
I use bootable DVD's or USB drives with Paragon Hard Drive Manager 15, Other Linux OS's running live from a DVD or USB.
For me Paragon is the best they only make it for Windows but you can boot to a USB or DVD and make system images of your Linux installs.
With a system image if anything goes wrong you can just restore the working image!

These are a few of the fixes I have had to use with Xubuntu/Ubuntu 18.04.2 "Bionic". The original 18.04 release was just really bad so I have been
trying each new one Xubuntu/Ubuntu 18.04, 18.04.1 and now 18.04.2 this one is close enough to make it run better than 16.04.2.

Note:
In 18.04 the apt-get has changed to just "apt" but apt-get still works. So if you use just "apt" and it does not work switch to "apt-get"

Software Center
The Software Center is gone (unless you upgraded from Xubuntu/Ubuntu 16.04 then you still have it)
To install the Gnome software center which just isn't very good.

1. Type or copy and paste these into a terminal window one at a time and hit enter after each one.
sudo apt update
sudo apt install gnome-software
sudo apt update
sudo apt upgrade

Steam Games
Install Steam or get it working again after an upgrade to Xubuntu/Ubuntu 18.04 from 14.04 or 16.04

1. Type or copy and paste these into a terminal window one at a time and hit enter after each one.
sudo apt purge steam
sudo rm ~/.steam if that doesn't work go to home/Your_Username/.steam (delete .steam folder)
sudo add-apt-repository multiverse
sudo apt-get install steam

2. After install type in steam to start it will update and restart once or twice.
Then log into steam all is good.
Now Steam should be on the game menu or you can create a panel or desktop short-cut.

Virtual Box - Virtual Machine Upgrade 5.3 to 6.0+
If you're using version 5.3 from Ubuntu 16.04 the upgrade to 6.0 is easy in Xubuntu/Ubuntu 18.04. Getting to 6.0 in Xubuntu/Ubuntu 16.04 was a pain!
Fix Virtural Box VM error only if you are running Virtal Machines. It's how most people run Windows or test other versions of Linux.

1. Type or copy and paste this into a terminal window and hit enter.
sudo nano /etc/apt/sources.list.d/virtualbox.list (or if VM is in the source list sudo nano /etc/apt/sources.list)

2. Then change: deb https://download.virtualbox.org/virtualbox/debian bionic contrib
To this: deb [arch=amd64] https://download.virtualbox.org/virtualbox/debian bionic contrib
sudo apt update
sudo apt upgrade

Any questions just ask.