Warning: Use with Caution
Developer Preview is an early version of Microsoft Teams yet to be released into general public release. The concept is to allow app developers easy access to new, unreleased features ahead of general availability so that they can take any steps necessary to ensure their app remains compatible.
However, it provides the normal user with a glimpse of new features coming to Teams. You can begin to discover new features, new behaviours as well as provide feedback to help improve Teams.
Switching to Developer Preview is only recommended for testing purposes only. There may be features that are incompatible with normal users and you should not rely on this release for main business functions. Not every feature will make it to general availability. Some can and have been deleted prior to release and if you have built a process around that feature, you have created a problem.
You can switch between developer preview and normal mode as much as you want. To access developer mode for Microsoft Teams, you must accept an EULA.
To begin, click on the ellipsis (…) and then go to About and choose Developer Preview

Accept the EULA and click Switch to Developer Preview

Teams will now reload into this mode. You will know you’re in preview when a little “P” bubble appears on your avatar

To exit developer preview, simply go back to the About section and uncheck the Developer preview feature.