May you tell me why?
Because this is Zentyal team decision to run internal (Samba) LDAP on port 389 and non-Samba LDAP on port 390.
Samba-LDAP should be available on port 389 with GSSAPI authentication but I doubt you can implement it from Alfresco.
Using standard authentication, you must use port 390.
Search this forum for more details or ask directly Zentyal if this is not enough clear.
And Do you mean I've to modify OpenLDAP port
No, I mean and say that you have to modify Alfresco LDAP client (or Alfresco configuration) so that you access LDAP on port 390.
or I've to change alfresco LDAP link similar to this:
ldap.authentication.java.naming.provider.url=ldap://localhost:390
I don't know where such change has to be applied on Alfresco's side.