• cd /usr/local
• git status
• Discard all the changes (unless you actually want to try to commit to Homebrew - you probably don't)
• git status til it's clean
• brew update

The right way to solve this is to enter:

$ cd "$(brew --repo)" && git fetch && git reset --hard origin/master && brew update

Found at http://discourse.brew.sh/t/how-to-upgrade-brew-stuck-on-0-9-9/33.

You can't do much with homebrew.

In my case fix this issue by updating ruby to 2.7 to install cocoapods v1.10.0.

rvm install ruby-2.7

Note: Use the specific version of ruby as per your macOS compatibility