The info object for the OpenAPI specification provides the ability to define a date-based or Semantic Versioning for the technical contract for an API. This version is often synonymous with the version of the API itself, but it doesn’t have to be, and can just reflect changes to the contract and not made to the API–I leave it to you to define what matters.
The reason for versioning the technical contract for your API is to be able to communicate change as it happens, so make sure that you accurately document and communicate around what has changed–don’t just make the change. The info version provides a record of change for any API consumer, but it also provides you the producer with a moment to pause and think about how these changes will impact the rest of the team producing the API, the consumers of the API, and any other stakeholders involved.