Not only can you create new records, but you can easily update existing records by sending a PATCH request.

When a record is created or retrieved using the API, one of the values that you will have is the id. This is a unique identifier for that specific record across the entire database. No two records across the whole database will have the same id value.

When issuing a PATCH request, you need to use the ID in order to tell the API which records to update.

An example of a PATCH request could look like this:

PATCH endpoint/companies(<id>)/items(<id>) Content-type: application/json

You can read more about the PATCH request on Microsoft Docs at https://docs.microsoft.com/en-us/dynamics-nav/api-reference/v1.0/api/dynamics_item_update.