Please help! Remove phone administrator

[Android Central Question]

My ex installed a tracking device on my phone and I just found it it's hidden under the app Wi-Fi when I try and uninstall it it tells me that I need to remove the phone administrator and when I go to click on that to deactivate it needs a password that he put in... I have an LG G6... Please help me he has access to my camera my microphone all of my messages everything basically he can even delete everything from my phone