How to remove administrator

I wanted some technical help in setting up the site and so I asked someone to make the changes for me. I registered him via add new users and gave him the title” administrator”

Now my site has 2 administrators, one is me and the other one is him. Now he has solved my queries and I either want to change his title from administrator to contributor or anything else or want to delete him, I am not able to do both. Any help on how I can delete or change his title?