Once an encounter is saved, us regular folks have no way of changing it.
Only those with knowledge of and access to the database can change it.
You might appeal to AC tech support to see if they can help.
This highlights another difference between government/payer approved & certified software, and a system that gives the Physician/Provider ultimate control over their treatment record.
There is a small but growing revolt to allowing payers to unilaterally dictate to Physicians/Providers, and this is one of those pain points that will only get worse.