How to adjust sleep goal?

You can't adjust the 8 hour sleep goal. The sleep coach can give you advice that deviates from the 8 hour sleep goal depending on how you sleep. Just give it some time. You can't adjust it.

So after a bit it will auto adjust the base up/down? So far I'm just getting a lot of "you aren't sleeping enough, add even more time to the 8 hours", "your sleep score is impacted because you didn't get 8 hours," etc.

It depends on the sleep quality. Just give it time. Not much to do since you can't adjust it.

Just to clarify, the base sleep goal provided by the coach is not necessarily 8 hours, it depends on your age (for example my goal is 7 hours 40 minutes). But I don't think it dynamically changes based on your sleep (at least mine hasn't). But individual day's recommendation can be under or over the base depending on several factors.

"Determining your nightly sleep need starts with an age-dependent baseline. The baseline sleep recommendation for adults under the age of 35 is 8 hours. This recommendation gradually decreases with age so that adults over the age of 65 start from a baseline recommendation of 7.5 hours of sleep each night." ( https://www.garmin.com/en-US/garmin-technology/health-science/sleep-coach/ )

Thank you for the correction. 

Working link:

https://www.garmin.com/en-US/garmin-technology/health-science/sleep-coach/