Hi Bob,
Since 19.04 does not receive security updates anymore, it would be wise to update.
Make sure you have the latest updates for your version ( you probably have if you updated regularly ) and after that you can update from the commandline with:
sudo do-release-upgrade
If that doesn't work, you could boot from USB if your computer is able to boot from an USB device anyway (your computer is a bit old, but i guess not that old, so I think it would be able to do so). In that case you could write the ubuntu-mate iso image to a USB thumbdrive and boot/install from that,
But if your computer is not able to boot from USB it will be DVD-burning time (unless you can clone the drive from another computer that runs a newer version of mate and you are not afraid to handle a screwdriver )
EDIT:
Oh I almost forgot another way with a reasonable chance of succes, (and ofcourse also chance of partially fail) but it will take some steps:
Be warned that this potentially can break your system
But on the other hand, you wanted a new install anyway.
Backup your homefolder (including hidden files/folders) just in case you screw up.
In 'software' disable all your PPA's so you end up with only the standard repositories
in /etc/apt/sources.list
change all occurences of disco
to focal
.
this is most easily done from the commandline:
sudo sed -i "s|disco|focal|g" /etc/apt/sources.list
sudo sed -i "s|disco|focal|g" /etc/apt/sources.list.d/*.list
sudo sed -i "s|old-releases|archive|g" /etc/apt/sources.list
sudo sed -i "s|old-releases|archive|g" /etc/apt/sources.list.d/*.list
Then take a deep breath and type:
sudo apt-get update && sudo apt-get -y upgrade
Don't be alarmed if there are some errormessages.
I would be highly surprised if there weren't any
If it bails out during one of these steps: Try to fix it, work around it (try to be inventive) or give up anyway and boot from USB or DVD for a fresh install.
If all goes well , the system will probably ask you to reboot.
If it comes up and runs: congratulations, you have been upgraded to 20.04
You can enable your disabled PPA's now (if they support 20.04)
If it doesn't come up: try linux bootrepair
Be warned that, on your venerable system, ubuntu 20.04 could be a tad slower than 19.04 depending on the processor you have (according to the specs, your processor could be anything from an AMD Sempron 145 to an AMD Phenom II X2 550) but not by much.
You will however discover a big slowdown if you have less than 2GB, so make sure you have enough RAM.