How to delete Ubuntu "try it out in parallel" version (20.04) after upgrading the original 18.04 partition (to 20.04)?

When you used "Try Ubuntu", nothing was installed. So there is nothing to remove.

If you installed Ubuntu in more than one location, you can usually delete the partition that contains the installation you no longer want, but this will not add any free space to your other installation. You would have to resize the partition after you delete the unneeded one. Make backups because if you make a mistake you will not be able to recover data easily, if at all.

If deleting or resizing partitions is not something you think you will be able to handle and you really want to use all of your hard drive for Ubuntu, reinstall Ubuntu and use the "Erask Disk and Install..." option.