cat /usr/local/directadmin/conf/mysql.conf

to get the login/password for mysql.

Type:
mysql -uda_admin -ppassword

where da_admin/password are the login/pass output from the mysql.conf.

Once in mysql, type:
DROP DATABASE da_roundcube;
quit

once the db is dropped and you’re out of mysql, type:
cd /usr/local/directadmin/custombuild
./build roundcube