Well Vojt, since you are asking about the process, I think I did something
similar going from testing to sid. First you would want to run a search in
your favorite search engine for "debian testing packages" Once you have
such a list, hopefully including packages from non free, also multimedia.
Open up in an editor like
sudo nano /etc/sources.list
Paste in that list at the bottom. I would then comment out with a number
sign your current stable list.
You would run an apt update -testing
After that an apt full-upgrade with sudo in front of all of these.
When I did this, I don't think I commented out my older list, so I ended
up with a combination. Most likely to endup on a newer kernel, you would
need to reboot after these updates. I hope that along with other advice
you are receiving will help you make the best choices for you.
Chime
