How to reset "users" screen settings through root-Fedora29

Hi all. I have a Fedora 29 based machine which I built in 2018. I don't use it very often as it is connected to a wide screen monitor which gives me neck ache! and I have another much older machine which I prefer. The other day I thought I would try changing the screen format (1920x1080) to something more like the old 4:3 versions. Unfortunately the Monitor rejected this and put up a error message on a black screen.I am now unable to switch it back. I can still get into a "root" screen and I suspect it may be possible to modify the "user" setting from there. Can anyone give me some hints as to how. Where are users settings stored for example?

Machine is using KDE Has Ryzen5 on Asus "Prime X470 -Pro" M/Board and Asus "Strix RX570" video card. Monitor is LG IPS225VX