Will any of my files be deleted while updating to 20.04

If your personal files are stored in your /home directory, there is very little reason to believe that any of your personal files will be altered or lost. If you have personal files written all over the disk, then there are no guarantees.

Do note that upgrading the OS will also update a lot of the software and their individual settings files. There is a chance that things may temporarily appear weird if a config file is doing something non-standard. If you run a MySQL database locally, be ready for the engine to be upgraded to MySQL 8.0, which is a pretty big change from the 5.x versions that were standard with Ubuntu 18.04.

Generally I recommend people have backups in case of hardware failure and/or human error. When upgrading an OS, I recommend people have backups just in case things go sideways.

The upgrade process was designed to not delete any of your personal files (so long as they are kept in the /home/yourusername/ directory, or on another, custom configured partition on the disk (that you (as a safety precaution) unmount and "unlink" (by commenting it out) in /etc/fstab beforehand)).

Yet, as things don't always work out 100% as planned, you could should do some safety preparations, to protect yourself from any unwanted consequences.

Before you perform the upgrade, prepare with two things:

1. An external drive onto which you back up all your important files.
   • This could be a portable USB HDD (probably the fastest to set up), or a Network Attached Storage, or a Raspberry Pi with a HDD attached, or within another full-fledged computer that you connect to on your local network.
     • Be prepared: if you end up using NFS or CIFS protocols for this, you need to learn to set the connection up with suitable file ownership and permissions. This task alone is a pretty attention-intensive one, so grant yourself enough time and capacity to deal with this. Even if setting up a backup solution delays your intended upgrade by two / three weeks, be in the know that it's the right thing to do, and you are making the right deviation from your original plans. You should rely on such an arrangement anyways, and could see immense benefits from it in the future.
   • Do a backup of your data and verify that you can access it on this drive and you can restore files from there, should the need arise.
   • You may choose to use the tar utility to back up all config files found in your /home/yourusername directory.
2. A live Ubuntu USB drive that you verify that you can boot from and initiate a "Try Ubuntu without installing" session
   • This could come useful if the on-disk OS gets a bit imperfect in the process, and you need to access the internal disk from a healthy, drop-in OS instance, let it be the system files- or any other partition.

If you are prepared in such ways, the upgrade can be an exciting adventure and a learning opportunity, instead of a gamble with too high stakes, otherwise.