Well, that's more a "demote" than "promote".
The instructions at
Promoting Replica to LDAP Master - Zimbra :: Wiki will tell you how to get from a replica to a master.
It also says to disable the old master at that time. So if you're trying to demote, you will need to make sure you have a new master in place, and then turn off the old master. You will want to remove the LDAP packages from Zimbra, and then re-install LDAP as though you are adding it as a brand new slave into the environment.