The Username, Password, and SSH keys can be changed:
Log into the UniFi controller.
Go to Settings - click the gear icon towards the bottom-left of the page.
Navigate to the System section.
Set the desired changes in the Network Device SSH Authentication section.
Click Apply Changes.
NOTE: After applying the changes, the adopted access points should automatically update with the new credentials.
If the new credentials do not work, then force the changes onto an access point by doing the following:
Log into the UniFi controller.
Go to Devices.
Click on an access point that needs updating - this will pop open a panel on the right side of the page.
Click the Settings icon - it looks like a gear.
Click Manage.
Click Provision.
NOTE: The status of the access point will change to Provisioning.