I'm currently working on Ubuntu 20.04 LTS but I encountered some problems with the compatibility to older software e.g. even with a lot of searching I wasn't able to resolve the errors for the following command
sudo apt-get install swig ipython python-dev python-numpy python-scipy python-matplotlib --install-recommends
which gives me the following errors
E: Package 'ipython' has no installation candidate
E: Package 'python-scipy' has no installation candidate
E: Package 'python-matplotlib' has no installation candidate
Therefore, I'd like to downgrade to 18.04 LTS. I think there is no other way than reinstalling 18.04 LTS on my system. My question is now, what is the best way to do this when I still want my Windows up and running. I'm asking because the last time when I installed Linux alongside Windows, I followed the recommendations of a website and my whole system was not bootable anymore. It took me a whole day to get it up and running again. My biggest question is, how to organize the bootmanager. It looks like shown in the picture now. 
If i read the output of the Grub Customizer correctly, there are two bootmanagers installed at the moment. One on /dev/sdb2 and the other one on / (root)? At startup, Windows starts automatically. If I want to start with Ubuntu, I have to hit the F11 key and choose the 'ubuntu' entry in the start menu. This is a behavior I can absolutely live with.
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