Can not extend existing account.

I am having problems with extending or renewing my minecraft realms account. All the wording suggests creating a brand new account but nothing describes extending or renewing an existing accout after the subscription runs out. The last time this happened to me I ended up with two unused unwanted realms accounts allong with my older account. I am due to extend, update or rewnew my subscription on my minecraft realms account but I am afraid to waste my money.  I was never refuned for the unused unwanted accounts  and that money was supposed to pay for future account extensions, updates or renewals. I returned to the same unclear wording that only suggest creating a brand new account. I just want to continue playing on my old account. I help my friends and family with their tech including helping with gameing account manegment and I can not see myself helping them waste their money due to a misunderstanding.