In recent Kdenlive version there is an issue where default project is set to 720p 30 frames which results in blurry render. It is easily fixable by setting 1080p 60 frames.

The issue is showed on those videos:

https://www.youtube.com/watch?v=H2s5Qjw-cFU

https://www.youtube.com/watch?v=1IesrRsRhIQ

However, my issue is: I want to change existing project! The fix works only for new projects but the old one uses old settings and I did some work and don't want to re-do it. Any pro tips?

Thanks!

Ah, found it. It is in Project>Project Settings. I was looking for it so long and couldn't see it, till someone pointed it out...
Facepalm