This button should do what it looks like it should do. It opens a drop down of actions that a user can perform on their user account. Should I go with "Setting" or "Settings"?
Setting is singular, so use it if you only have one thing to set.
Settings is plural, so use it when you have multiple things to set. This is likely to be the case most of the time, so when in doubt, use 'Settings'
There seems to be several things to set. So Settings, plural.