Remove old Admin?

Just bought an iMac (second hand) from a friend,it runs el capitan.
I need to find out how to remove his account details and replace them with mine? I can find no way to do this? Any advice would be welcome.

Here are the steps that a seller of a used Apple computer should follow before selling:

https://support.apple.com/en-us/HT201065

What you mostly want to do at this point is #5 in the link (erase & reformat the HD...and reinstall the OS).

After this is done...you may also want to try signing into iTunes or the App Store and see if you have any issues. If you do...you may want to go back to your friend and have them sign out of items 2, 3, and 4 in the list.

HTH,

- Nick

Make a backup first ... preferably a clone.

Cheers ... McBie

Thank you both for your help and advice.