How do I reset my admin password while still logged into my admin account?

To clarify, I really mean RESET. I changed it while I was updating a few things in the Systems Preference. When I went to click on the lock icon to make changes in Users & Groups, I was prompted for my password, so I typed in the password I thought I changed it to but no go. I must've made some typing error x2 when changing the password. Also, the previous owner of this computer never disconnected this device from his Apple ID which means I am currently unable to connect it to mine, though my FileVault is off anyway and without an admin password, I can't enable it.

Anyway, I'm stuck because I don't want to shut off my computer to reset from the Recovery Partition on the chance that I am also inept enough to screw that up as well and ultimately being locked out of my account all together.

My first thought is to use Terminal, but I know nothing about what to input since I am relatively new to Mac computers. resetpassword doesn't work.

How do I solve this???