Or login via SSH as root and:
cd /usr/local/directadmin/custombuild
there edit the options.conf and change php 7.3 to php 7.4, don't forget to set php_ini to yes to get a php.ini file too.
Save the file and then run:
./build update
./buld update_versions
I would still look at the link given above, to learn what you're doing.