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I accidentally upgraded Ubuntu from 16.04 to 18.04. Now I can't log in to the system. But through the PE system, I can see that the previous file is still on the hard disk. Is there any way to get 18.04 back to 16.04? Or if I want to reinstall the system which paths should I save from the disk?

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marked as duplicate by pomsky, karel, N0rbert, Zanna, Charles Green Dec 5 at 18:51

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There is no 'roll back' since there is no Windows Product Key to preserve. There is only reinstall.

Save your data, wherever you stored it. Reinstall the rest.

Love this question!

Just reinstall and get good at it! (That is how many Ubuntu superusers know so much.) But here is an easy way to back up your stuff in the folder when you do, for times like this...

While installing from the liveboot USB, mount your original hard disk where yout home (or Ubuntu) is, right click, "open the terminal", and do something like:

sudo mv /home/YOURUSER /home/myoldhome

Then after the install, your files will still be there.

Although you cannot restore directly to 16.04 from within 18.04, you can install 16.04 in a separate partition and then add it to Grub as an extra boot option, like so.

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