use the ‘df’ command to confirm that boot is full
cd /boot
ls -la //to see the files

You will notice a lot of files that seem to have old versions. It’s safe to remove all but the most recent of these files.

sudo rm *.0-23-* //to remove the version numbers that are unnecessary
repeat the process until you only have one latest version left. If you delete the latest version you will seriously mess up your system so be careful.

a final ‘df’ will confirm that you have freed enough space