New mac os available.  You try to install but you run out of space.  It will fail and your mac will be hosed.  You could press command + R while booting and get into OS Utilities, but it doesn’t allow you to free up space and delete files individually.

Solution:

1. boot into live usb (say fedora).
2. Then install hfsplus-tools (see this if you encounter the error mentioned in that post).
3. Then run a fsck.hfsplus with -f on  /dev/sda2 or whatever
4. Now mount your drive, e.g. mount -t hfsplus -o force,rw /dev/sda2 /mnt/mac

Now your drive should be writable, i.e. you can delete files.

Other than data files, you can consider deleting applications.  The Applications directory is hidden in the “Recovery Items” folder.