How do I delete a gmail account from my iMac

My iMac changed my g-mail account and this has caused havoc with all devices. I still have my old gmail account and would like to keep it. My iMac changed some of the letters of my user name from lower case to upper case on my iMac. Then as I was away from my computer for 2 weeks I was locked out of account on my phone. How do I delete this changed account on my computer?