Introduction
Open System Preferences. Click Users & Groups. Pick the account that you need to set as an administrator. Check the box "Allow user to administer this computer".
Instructions
2) Click Users & Groups.
3) In the bottom left-hand corner, click the lock icon to unlock the preferences pane.
4) Type a username and password with administrative privileges on your Mac. Then, click Unlock.
5) Check the box next to "Allow user to administer this computer".
OUTCOME
The selected user will now have administrator access.