I presume you have the original project file ... I'd just resume using the version it was created in and export from their. And btw ... updating a project is not considered a best-practice. Most of us keep previous versions "up" as long as we need to for finishing of older projects. I'm just past using 2014 now, and don't have it loaded. I still have 2015.2 and 2015.4 ... and note, 2015.3 SHOULD have been named 2016 ... it was a complete version/build update from 2015.2.
To download a previous version ... as shown below ... go to the "Find Additional Apps" tab circled here ...

... and ignore the "Previous Versions" thing I point to. Simply go down the list below, and by Prpro, click the "Install" button, which will drop a list down of previous versions. It gives you the first release of each version, and after downloading/installing one, you can then update that to the last release of that version.
Neil