What action can be used within a flow to update existing records?

To update existing records within a flow, the appropriate action to use is the option that specifically targets existing records for modification, which is updating. The "Update Records" action is designed to facilitate the identification and modification of one or multiple records that already exist in the Salesforce database. When this action is selected, you can specify criteria to locate the specific records to be updated, as well as define what fields should be modified and their new values.

The other actions serve different purposes: "Get Records" is utilized for retrieving data from existing records but does not alter anything. "Create Records" is meant to add new entries to the database rather than modify existing ones. Lastly, "Delete Records" is specifically for removing records altogether, rather than updating them. Thus, the action most aligned with updating existing records is "Update Records."