Apps, especially beta ones, sometimes appear in the wild in the form of .apk files that leave people scratching their heads wondering what they can do with those files. Well, the answer is actually very simple. You just need an extra app that will install these .apk files and turn them into apps, much like an installer in a computer.
The first thing you will need to do is to move these files into your phone. There are many ways you can do that like connecting your phone with your computer and selecting the “USB Connected” (or similar) option. Apps like tAttachApkInstaller will also let you download the files directly to your phone from Gmail.
Another solution is to use a file manager like ASTRO. That way, you can simply navigate your way to the downloaded .apk files and install them directly from the file manager by tapping on them. Know that you will get a warning when you try to install an .apk file, regardless of the way you use. That is because .apk files might come from untrusted sources and harm your device so make sure you only get such files from reputable websites.