How to upgrade Ubuntu Server from 18.04 to 20.04?

Hi there, I am trying to upgrade the Ubuntu server 18.04 LTS running on my Rock Pi4B to 20.04. I found this link I encounter some error even after I change the initramfs.conf to “COMPRESS=xz”. The error message is “The upgrade needs a total of 110M free space on disk’/boot’. Please free at least an additional 77.1M of disk space on ‘/boot’. you can remove old kernels using ‘sudo apt autoremove’ or set COMPRESS=xz in the /etc/initramfs-tools/initramfs.conf to reduce the size of the initramfs.” can some pls help on this issue. Thanks.