brew install old versions

cd to Homebrew's formula directory

cd $(brew --prefix)/Homebrew/Library/Taps/homebrew/homebrew-core/Formula

2) Find desired commit (version 63 for icu4c) to checkout

git log --follow icu4c.rb

3) Checkout to a new branch

git checkout -b icu4c-63 e7f0f10dc63b1dc1061d475f1a61d01b70ef2cb7

4) Reinstall the library with the new version

brew reinstall ./icu4c.rb

5) Switch to the reinstalled version