PMI usually released an updated version every 4-5 years.
The PMBOK 5th edition was released in 2013 and the 6th edition was released in September 2017. This means that you won't get a newer PMBOK version until sometimes in 2021 or 2022. Therefore, if you are preparing for the exams anytime now to anytime before 2021, you will be using PMBOK 6th edition.
As for the exam content outline, the current version was published in June 2015 and so it is time to publish a newer version in June 2019. I think the update is needed in lieu of the new exam platform (from Prometric to Pearson). This means that if you are taking exams after June 2019, you will have to refer to the new exam content outline. Some of the exam questions come from the "Performance Domain" section and so it is important to ensure that you read them.
Hope this helps