macOS: Setting up an administrative account from a user account without admin password
Apple has a guide that covers all the permutations and combinations (iCloud / FileVault / forgetfulness / no password at all)
- https://support.apple.com/en-us/HT202860
If you have FileVault turned on, you likely need to reset the password with Recovery boot.
- Reset admin password without the old password
If you don't have FileVault turned on, you can simply make a new admin account and then use that user/password to make any other non-admin accounts back into admin accounts.
- How can I get admin access to a Mac without knowing the current password?
The above covers my favorite way - remove .AppleSetupDone and make a new account, but some people prefer to not worry about a new admin user and just do the full reset with ddcl
You can no longer just force the reset from the OS that's running - that's the same as defeating the reason to have a password and Apple has patched all the easy / known ways to do that I believe.
If you have setup the admin account using an Apple ID, you can reset the login password using it. From the macOS user guide article, Reset your Mac login password:
Reset your login password using your Apple ID
If you associated your user account with your Apple ID, you can use your Apple ID to reset your login password.
On your Mac, choose Apple menu > Restart, or press the Power button on your computer and then click Restart.
Click your user account, click the question mark in the password field, then click the arrow next to “reset it using your Apple ID”.
Enter an Apple ID and password, then click Next.
Follow the instructions to reset your login password.
The user guide is for macOS Mojave. I'm not sure if the same works for macOS Sierra, but you can give it a try.