$ brew install php@7.4

・・・

Error: php 7.3.9_1 is already installed
To upgrade to 7.4.5_2, run `brew upgrade php`.
==> `brew cleanup` has not been run in 30 days, running now...
Removing: /Users/kanehiroyuu/Library/Caches/Homebrew/libunistring--0.9.10.mojave.bottle.tar.gz... (1.4MB)
Removing: /Users/kanehiroyuu/Library/Caches/Homebrew/nodebrew--1.0.1.tar.gz... (26.3KB)
Removing: /Users/kanehiroyuu/Library/Caches/Homebrew/terraform--0.12.20.mojave.bottle.tar.gz... (15.3MB)
Removing: /Users/kanehiroyuu/Library/Caches/Homebrew/portable-ruby-2.6.3.mavericks.bottle.tar.gz... (9.0MB)
Removing: /Users/kanehiroyuu/Library/Logs/Homebrew/terraform... (64B)

エラーがでたな。

これで良いらしいな

$ brew upgrade php

$ php --version

PHP 7.4.5 (cli) (built: Apr 23 2020 01:10:12) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.5, Copyright (c), by Zend Technologies

アップデートされたな！