By the way, the document Booting the previous OS version shows how to boot previous version (Rollback) with GRUB. It includes a demo video, too.
Some computers, such as Acer Nitro AN515-54 with Intel® RST feature may boot failed after upgraded from EOS 5.0.x to EOS 5.1.0. “Rollback to EOS 5.0.x with GRUB, boot into the system and upgrade to EOS 5.1.1 again” can avoid the the issue.