There could be various reasons when you want to reset the Mac admin password.You may have forgotten the password and stuck at login screen, or you may need to upgrade your Mac, remove any software, make changes to system preferences or create a new user account- all of these tasks require you to verify with Mac admin password. If you know the old password and want to reset it for any security reason then the task is simple.
1. Just go to System preferences and open Users & groups.
2. Select user account you need to change password for and click change password button on the right.

But f you have forgotten your Mac admin password you can not reset it from system preferences rather you need to go to recovery mode or Single user mode to change the Mac admin password.
In Recovery Mode:
1. Hold down the keys command+R together immediately after turning the Mac on.
2. Choose Terminal from utilities menu at the top and type command "resetpassword"
3. Select the volume containing the user account and select then select the usernme.
4. Click continue and type new password.
5. Restart when done.

If you are unable to boot your Mac from recovery mode due to some reason like corrupt or missing partition you can reset the mac admin password from single user mode otherwise.

The article https://iguruservices.com/support/reset-mac-admin-password/ describes about all the different methods of resetting Mac administrator password in different version of macOS or Mac OS X.