Old Phone Number

I bought my Ring Doorbell (version 1, I think) quite a while ago.  I registered my User Name (but THINK I did not register my device). Having not used it, I wish to install for my step-daughter.  Trouble is, I went through the Reset password but, on trying to log in, it wants to send a code to an OLD mobile phone number (I never changed it once I'd got a new mobile number).  How do I reset my Mobile number if (a) I cannot login nor do I have a Ring Doorbell set up?