How can I change the name of the user on my iMac??

System Preferences > Users & Groups, then ctrl-click on the User (Current User-left side) and choose 'Advanced Options', you can change the 'Full Name' in the settings that appear.

To change the user's Full Name, which is seen at login and on a variety of other system panes, navigate to System Preferences and select the User & Groups pane. Click the lock icon and enter your password to unlock. Control or right click the user you want to rename, and click "Advanced." Find the field labeled "Full Name" about half way down the pane, and enter a new full name for your user.

If you're interested in changing your user's short name, which is the same as the name of your home folder, the process is significantly more complex and can break things that rely on your home folder's name, such as Dropbox. However, if you're backed up and comfortable with that scenario, you can follow the instruction "Change the account name and home folder name" in Change the OS X user account name and home folder name on your Mac - Apple Support.