how to remove the root certificate

Depending on how it got installed and by whom - for instance, your iPhone is in any way managed by your company or school - it may not be removable, at least not without assistance from your IT department, but try this:

1. Open Settings and select General.
2. Select Profile (if you don’t see any profiles, there’s nothing to delete).
3. Choose the Profile you want to delete.
4. Select Delete Profile.
5. Enter your pass code (if prompted).
6. Select Delete one more time to confirm.

If you find nothing there, similarly go to Settings -> General and select VPN & Device Management and check for profiles there.

Regards.