If you've installed the release version of Windows 10 on your machine, but want to switch to the Insider Preview builds, you don't need to reinstall Windows. Just tweak this one setting.
This week, Microsoft released its first preview build since Windows 10's release, complete with a few memory improvements, title bar colour customisation, and bug fixes (along with a few new bugs, of course). You've probably been hearing about it everywhere, and if you want to try it out on an existing Windows 10 machine, you just need to tweak one setting.
Head to Settings > Update and Security > Windows Update > Advanced Options and click the "Get Started" button under Get Insider Builds. If it's greyed out, you may need to switch to the Enhanced or Full setting under Settings > Privacy > Feedback and Diagnostics > Send Your Device Data to Microsoft. (You can read more about that setting here.)
Remember, the insider builds collect more information about your usage than the release versions of Windows 10, and are likely to have more bugs -- so we don't recommend using it as your daily driver. But if you need to run the preview for any reason, it's easy to migrate an existing installation.