Cant upgrade from mint 18 to newer

I had to upgrade my parent's pc which was running a linux mint 17.3. It was no problem to upgrade 18, but then mintupdate app did the sam again and again, asked to install 18, then checked some things, all 'already the newest", done. I did this around 5 times, no option for 19 upgrade appeared, not even with apt dist-upgrade, which showed nothing to upgrade. For all packages apt writes nothing to upgrade. For apt install mintupdate it writes no newer relase, so ive purged it, then tried to reinstall, and writes mintupgrade not found, using the same command line that i used on 17 before the upgrade. Now how can i continue upgrading to newer mint?