If you would like to change your root password in MAMP on your Mac, please do not look all over the internet to find partial answers like I did. Instead, let me just tell you the complete way to do it that worked for me.
1. The first step is to open Terminal, paste the following code in, and change the <NEWPASSWORD> to the new password you would like (leave out the <>). Hit Enter. Please know that MySQL must be running, so stay logged in to MAMP, while you do this.