Try this:
# Includes
# shellcheck source=/etc/hestiacp/hestia.conf
source /etc/hestiacp/hestia.conf
# shellcheck source=/usr/local/hestia/conf/hestia.conf
source $HESTIA/conf/hestia.conf
# Remove apache2 from config
sed -i "/^WEB_PORT/d" $HESTIA/conf/hestia.conf
sed -i "/^WEB_SSL/d" $HESTIA/conf/hestia.conf
sed -i "/^WEB_SSL_PORT/d" $HESTIA/conf/hestia.conf
sed -i "/^WEB_RGROUPS/d" $HESTIA/conf/hestia.conf
sed -i "/^WEB_SYSTEM/d" $HESTIA/conf/hestia.conf
# Remove nginx (proxy) from config
sed -i "/^PROXY_PORT/d" $HESTIA/conf/hestia.conf
sed -i "/^PROXY_SSL_PORT/d" $HESTIA/conf/hestia.conf
sed -i "/^PROXY_SYSTEM/d" $HESTIA/conf/hestia.conf
# Add Nginx settings to config
echo "WEB_PORT='80'" >> $HESTIA/conf/hestia.conf
echo "WEB_SSL='openssl'" >> $HESTIA/conf/hestia.conf
echo "WEB_SSL_PORT='443'" >> $HESTIA/conf/hestia.conf
echo "WEB_SYSTEM='nginx'" >> $HESTIA/conf/hestia.conf