Whenever you hit an API operation, you'll notice the API version is returned as a header value named X-Tactic-Version.
If you want to target a specific version, use media type versioning by specifying the version in the standard Accept header. For example to target version 1.0 you'd do:
$ http GET 'https://api.gettactic/user' 'Authorization: Bearer ACCESS_TOKEN' 'Accept: application/vnd.tactic.1.0+json'
If you don't specify the Accept header, or you specify the generic application/json content-type, the default API version implementation will be used.