The <overwrite> directive is usually the best approach to update a controller's user tag.
You can create 1 or more controller user_tag configuration packages as needed to include a custom tag value.
Here is an example:
<controller> overwrite
os_user1 = abc
os_user2 = xyz
</controller>
5. Deployed the package to the target robots and the user tags were updated.
Note that you can create a group/groupings of robots in the IM if you have to update a large number of robots and then deplpu the pacge to the target groups.
There is an old discussion about it here in the community as well.
https://community.broadcom.com/communities/community-home/digestviewer/viewthread?MID=757569
Lastly, there are some important notes on updating user tags on older robot versions, if applicable in your environment, e.g., v7.70, 7.80 here:
Infrastructure Manager Groups Node (How to create and work with IM Groups)
https://knowledge.broadcom.com/external/article/10343