System update is connected to cron, which checks for updates. If you only set site settings for updates, it will follow the stable versions upgrade path. If you have installed Developer Module in addition and you set it to receive Alpha/Beta version updates, it follows that upgrade path.
The Wiki gives following information:
There are several update channels - stable and beta. By default stable is used. It's possible to change in Studio > Developer > Settings > Update channel. If Developer module can't be installed then it's possible to change update channel by running the following SQL query:
UPDATE `sys_options` SET `value` = 'beta' WHERE `name` = 'sys_upgrade_channel';
Then clear DB cache in Studio > Dashboard > Cache block