forgot login and password

[sumer]

i) downloaded an OS and mounted it on a micro SD

What OS have you downloaded and how did you mount it on a microSD?

I tried dietpi and used balena etcher. What happens is basically: it goes through all the steps and when I get to final reboot. I takes me back to my old log in screen (with the old username etc)

[sumer]

In the meantime I managed to reboot it with Manjaro (from Sd card). It turns on nicely but as I remove the sd card and restart the computer it goes back to to the old OS....

[jezek]

I tried dietpi and used balena etcher. What happens is basically: it goes through all the steps and when I get to final reboot. I takes me back to my old log in screen (with the old username etc)

Speaking from my own experience, I wasn't able to boot every image from the wiki page. Some just didn't work. It was some time ago, maybe now all work. I can only say, that the Manjaro images are working. One week ago I installed the Manjaro KDE onto SD card to see what's new and it booted. I use on my Pinebook 1080p the Manjaro Sway image, I think it's the fastest and most responsive image for "old" pinebook. It can even play (supported) 1080p video without lagging.

In the meantime I managed to reboot it with Manjaro (from Sd card). It turns on nicely but as I remove the sd card and restart the computer it goes back to to the old OS....

Congratulations. So now you have 2 options.
1) Use information form previous posts to reset login/password
2) Bacup the data on EMMC and install new OS from scratch to EMMC. You should find he instructions on wiki/forum (If you're using manjaro, they have "manjaro-arm-flasher" and "manjaro-arm-emmc-flasher" tools, which makes installing different manjaro images easier).