So, I just installed Ubuntu 16.04 (will upgrade to bionic then focal) and RetroPie 4.6 on my Raspberry Pi 3B and I am trying to upgrade. I have removed a lot of unneeded software and whenever I try to upgrade, I can't because free up space for the /boot_ub partition because there isn't enough space. 21.3MB used and 44.6MB free. I tried some tutorials to remove old kernel modules to free up disk space, but there aren't any to remove.
The /boot has plenty of space, 212.9 MB left.
But it is saying /boot is low on disk space.
I need to upgrade to 20.04 because the stupid boot_ub is too full and I don't know what I can do. Help is appreciated. 
EDIT: Would it help to remove RetroPie? I installed both OSs with PINN (enhanced noobs)