The fast ring gives an opportunity to find bugs in the OS, and gives you early releases of upcoming versions. The slow ring gives an opportunity to try a more stable version of an upcoming release.
Alternatively, go to Settings > Update and Security > Windows Update and click on Check for Updates. The version will not come right away, but when the update is ready, you will get a notification to set a few privacy settings and then proceed to install the update later or install it right away.
