Per MSPowerUser.com – If you want the Windows 10 Fall Creators Update right NOW in three different ways: using the Update Assistant, using the Media Creation tool, or by clean installing using an ISO. Installing with the Update Assistant is probably the better choice as it is easier to use, but you can also get a fresh start by downloading the ISO using the Media Creation Tool and clean installing using the ISO.
You do not have to do any of the above options. The Fall Creators Update will eventually rollout to your PC. You do have to have Windows 10 on your PC in order to qualify for the update. Typically, what I do is use the “Update Assistant” option on my laptop to see how the install performs and how long it takes, before I install on my main computer. From my experience, these updates can take a considerable amount of time; and, as with any major update (or upgrade) problems can be expected.