On an older version of DirectAdmin, the proper process to switch to Nginx_Apache from either Nginx or Apache is as follows:
SSH into your server, sudo or su to root and execute the following commands:
cd /usr/local/directadmin/custombuild
./build update
./build set webserver nginx_apache
./build nginx_apache
./build rewrite_confs
While those commands work fine on modern versions of DirectAdmin, the up to date method is as follows:
SSH into your server, sudo or su to root and execute the following commands:
da build update
da build set webserver nginx_apache
da build nginx_apache
da build rewrite_confs