Can I reset root password?

If you go for the option of save all files to external drive, then your clean install gives option to wipe everything off the laptop, install os, then load what you want from the external store

I guess you tried the command in a terminal: sudo passwd and it didnā€™t work?

I understand how you might be hesitant. First, you need no permissionsā€“itā€™s your laptop and your choice. Donā€™t worry about GRUBā€“most distros construct that for you in the installation process. Your main concern is to decide on an external storage plan and execute it.
My practice is to copy the Home folder to a blank USB/CD/DVD or external hard drive, to keep the data safe. Itā€™s faster than trying to pick specific files to save.
Once your data is safe, boot from the USB containing your chosen distro. Pick the ā€˜erase and replaceā€™ option on the hard drive you have chosen. Most distros offer this. Many often allow you to format the drive as well, which might be a wise idea.
Once your installation is complete and you have rebooted, with new passwords and users identified, load the storage medium where you parked the Home folder and repopulate your dataā€“most distros have the same folder structure. Open two incidences of your new file manager, one for your new drive and one for the storage drive, and just drag and drop.
Akito, Mina, Andy2, Rosika: please append any corrections or additions so that Tangra has the benefit of your wisdom as well.

Thank you for the detailed reply, berninghausen! Ill try it.
Im going to install Ubuntu again. Hope will not have problem with the GRUB.
I have tried Ubuntu, Kubuntu and Debian.

Yes, I tried in terminal.
I wrote the commands, but wasnt asked to write my password to confirm the commands. Just nothing changed,

Best of luck! We were all there once.

Ubuntu is a good choice but you may also want to look at Mint. Iā€™m thinking it would work well for you in your situation. Good luck and let us know how you make out.
How big is the HD in your Machine?