That's quite short, but of course, spiky0011 is right.
A little more verbose: look up an howto for chroot via your favourite search engine (the *buntu ones usually are quite detailed) and then check if you have some live/repair/install system on a CD or USB device with a kernel version very close to your installed one. Boot that system and chroot to the installed system (in a terminal, that is; you will be the root user of the chrooted system) and type passwd. After setting the new password, type exit (will leave the chrooted system and return to the one from the CD/USB). Then shutdown the system properly and boot into the installed system.
Another idea: If you think you remember the passwords but the system just doesn't accept them (any more), check the keyboard settings. A lot of installers use US keyboard for installation (or as a default) but the ones used in 90% of locations all over the world are localised ones. So if you didn't set a local keyboard during installation, after booting up the first time, the key map is likely different than during installation. (That blooming Ubuntu 12 messed that one up pretty badly for me.)
Last edited by Ratamahatta; 04-20-2020 at 06:44 AM.
Reason: added info
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