How to change a user to moderator

I am tying it change a user to moderator and when I go to their profile and then down to rank title I click on moderator and its keeps giving me box asking to select with users password I want to change. The user who rank I setting isn't even a choice and I am not trying to change any passwords in the first place. So what the problem here. Help please.

Well the panel asks your admin password not the users password.
If you have an group named moderators you can simply add him to that group so I don't know what are you doing with the custom title. If you don't have a group for moderators you should make one although I believe that by default you already have this group unless you deleted it. Anyway if I am wrong or you deleted the moderators group here is how to make a new one, go to the admin panel =>Users & Groups=>Groups=> Groups Administration from there you can create a new group select a legend color for it, and edit the permissions of that group of course.