It is now possible to upgrade LMDE 4 to version 5.
First, refresh your cache and install the Upgrade Tool by typing the following commands in a terminal:
apt install mintupgrade
Then type the following command to launch the Upgrade Tool:
Follow the instructions on the screen.
When the tool is done and the upgrade is successful uninstall it and reboot your computer.
apt remove mintupgrade
This is a major upgrade. It can take several hours.
You will be asked to be up to date and to prepare system snapshots. Do not rush, do not take shortcuts.
Don’t hesitate to seek help if you have questions or if you face problems with the upgrade.