How do I change an ordinance date to the earliest date it was completed?
The only way you can change it is to find the duplicate entry in Family Tree for that person that has those ordinance dates and merge it into the record you know about.
Otherwise all you can do is post a request here in Community under the Temple section to Support with the person's name and ID number along with the information you have about the earlier dates with proper documentation and see if they are able to confirm the information, find the ordinance record and put it on the person.
It is not always possible to get the earliest date on if there is not sufficient proof for them.
Also, be aware that in the days when confirmation was required to be on the same day as baptism and initiatory was required to be on the same day and endowments, the dates of confirmation or initiatory were not recorded and appear in Family Tree as Completed. If later ordinances were done, the later date overrides Completed when records are merged and there is no way to change that.