I cannot, as I haven’t ever done it, so I cannot say the correct way.
What I have understood is that you have to stop upgrading your system from the non stable releases and wait of a stable release that is ahead from your currente release.
Then edit app.yml file and edit it to use stable release instead of tests-passed.
Then perform a complete upgrade.
If I understand it well, there is not 3.2.0 stable release yet.
What was announced is a 3.2.0-beta5, so may be there would be one in the near future.
What I have installed right now is a 3.2.0-beta5-dev (67244a2318)
Where can be seen the current releases published under stable branch?
EDIT: I have to rectify, it seems 3.2.0 is already published and is the same version than 3.2.0-beta5