ULib updates access levels anytime ULX
adduser/removeuser is used. No need to reload.
If you aren't using ULX, you'd need to write your own function to do that action.
Unless Owner is a group (or UTeam team) you created, there is no Owner group by default in ULib or Gmod.
There are two users.txt files. One ULib creates/updates; /data/ULib/users.txt and one that Gmod uses (and ULib imports) for (all) Lua scripts to determine admin access on a server.
You'd need to use "ulx removeuser" to remove yourself from ULib's by default, edit the Gmod's /settings/users.txt to not include yourself, then rejoin the server.
To my knowledge, /settings/users.txt can't be reloaded dynamically and may require restart of server for its changes to take effect.