It's possible to keep up to date by downloading installers for new versions of software from the Apple downloads site:
... if your ASU has just been uninstalled, an update using one of those installer bundles would most probably also reinstall ASU.
If your ASU is damaged in some way, however, that can lead to some troubles using the installers downloaded from the downloads site. For example, see the following user tip:
... so it might be worth investigating a repair install of ASU (as per the instructions in that user tip), if you've got iTunes installed and want to keep that up to date.