Upgrade Laravel Valet to PHP 7.4

First you should upgrade your machine to PHP 7.4, it you haven't checkout this great guide from Brent 

the short version is to do these steps using homebrew:

Update Brew

brew update

Next upgrade php

brew upgrade php

At this point your machine should be using PHP 7.4 but Laravel Valet may still be using PHP 7.3.

Upgrade Valet:

Update composer

composer global update

Next update Valet

valet install

Now to tell valet to use the latest version of PHP

valet use php

this tells valet to unlink php 7.3 or whichever version you have and use the latest version i.e 7.4

Or you can specify the version to use:

valet use php@7.4