Open the Activities overview and start typing System.

Select Settings ▸ System from the results. This will open the System panel.

Select Users to open the panel.

Press Unlock in the top right corner and type in your password when prompted.

Click on the user account that you want to delete under Other Users.

Press the Remove User... button to delete that user account.

Each user has their own home folder for their files and settings. You can choose to keep or delete the user’s home folder. Click Delete Files if you are sure they will not be used anymore and you need to free up disk space. These files are permanently deleted. They cannot be recovered. You may want to back up the files to an external storage device before deleting them.