You need to explain in more detail, I'm afraid.
> we are currently uploading our users to moodle as Ldap authentication and this works fine,
Basically, you followed
https://docs.moodle.org/en/LDAP_authentication, right? Anything more, special features? Do you pull users executing auth/ldap/cli/sync_users.php?
> but we want to automate the process of user creation with read from external database, we managed to do that
What is the external database? Where does this method create users, in the LDAP or in Moodle?
> but the authentication of created users are set to database,
OK. It seems that the external database authentication is in addition to the LDAP. Yes, Moodle can support many authentication methods at the same time.
> we want them to authenticate using LDAP , is there a way for that?
Technically, you can change the field 'auth' in the mdl_users table to 'ldap'. But then Moodle will look for their passwords in the LDAP. If that is what you want.