How do I upgrade to PHP 7.4 from 7.0?

Here you go:

https://computingforgeeks.com/how-to-install-latest-php-on-debian/

I have used the Sury PPAs in the past and they are of generally high quality. However, when you go to upgrade your system to Debian 10 or the Debian after that, you may have to revert back to the "stock" php(1) to get the upgrade to work properly.

YMMV!

-- sw

I have used the Sury PPAs in the past and they are of generally high quality

I second that.