I am unable to change my Apple ID and reset my password

Activation Lock?

If so, Activation Lock is intended as a theft deterrent, and is intentionally difficult to bypass. We can’t help, and Apple—absent sufficiently-convincing proof of ownership—won’t help.

If you purchased the iPhone used, contact the seller to clear Activation Lock, or for a refund. If the seller has disappeared as is all too common, the iPhone is probably lost or stolen, and you got scammed.

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

"Some SECURITY thing" isn't really cutting it in terms of a technical description with which we can work. ;-)

If you see an actual error message then quoting it would save us a lot of guessing.

If it refers to this then read the article about that feature About Stolen Device Protection for iPhone - Apple Support

and how to work with it.

If it is Stolen Device Protection, you will need to wait the hour that it tells you before making any changes. A screenshot may be helpful do determine what the SECURITY thing that you are seeing.