The Android Oreo update has (finally!) started rolling out to the Galaxy Note 8.
If you purchased your Note 8 from a carrier, then when you receive the update will depend on them, not Samsung. If you don't have automatic updates set up for your device, you can see if it's available for you by going into the Settings on your phone followed by "Software Update."
These things are typically rolled out in waves, so if you don't see it right now, try again over the next few days. Keep in mind that installing it is going to take your phone out of commission for a few minutes.
Installation requires your phone to restart. You can also schedule the installation to happen at a time you choose in that Software Update menu, so you can set it to happen at some time like 2am while you're asleep and wake up to a brand new version of Android in the morning.