Trying to set up keychain; verification text delivers to old number

I am trying to set up keychain across all of my devices, and while I know the correct 4 digit passcode, the verification text gets sent to an old number. This is not my current number and I have no access to this (nor can I find it as the number listed anywhere in my iCloud settings. I can't figure out how to make the verification number send to my current phone.

I have consulted this webpage (Get help using iCloud Keychain - Apple Support) but don't see the 'Account' option within 'iCloud" on my iPhone, nor does anything happen when I I try to open 'Options', when trying to resolve the issue on my Mac.

Any help is very much appreciated!