There may also be nice hacks if you search around, that allow you to get the user-switching icon or a keyboard shortcut setup to do a fast-switch, or trigger a switch or logout by tweaking a call to CGSession, although that may have been possible only in older OSX versions. If you would rather use fast-user-switching, which I personally don't like because it can often really slow down your computer, then you can set up an "logout user after x minutes of activity" option in the System Preferences -> Security & Privacy, which will return you to a login screen. Single user mode is a special startup environment, which boots Mac to a terminal-like interface where users can run commands to do what they want on Mac, such as repairing startup drive or creating new admin account etc. Of course, for either of those options to work, you have to enable Remote Login and Remote Management in "Sharing" under the System Preferences. Create new admin account in single user mode when cannot sign in Mac. Here's a really easy-to-read article about this.
MAC CHANGE USER TO ADMIN MAC OS
You must login to the computer and not simply "Share screen" from Finder. This method utilizes System Preferences to privilege a user account to admin status, it works to convert any standard user account into an administrator level account in any version of Mac OS X, whether it is called Mac OS X, macOS, or OS X does not matter, the procedure is the same.
As long as it is ok to forcibly log-out the user, you just need to to logon as the local admin using Apple Remote Desktop (available in the App Store) or ssh (through a terminal), and kill that user's loginwindow process.