How do I change my old acct on icloud to my new Apple ID?

This is my dilemma....I cannot purchase new apps or anything because I don't have enough storage.....so I tried to upgrade my icloud account and it keeps telling me to enter my password....the problem is....it's showing my old email acct that is no longer in use for a few years now.....and no password can be remembered.i do have a new emIl account for yahoo and I tunes and my Apple ID are all the same now...except for my icloud account. I tried to delete but it will not let me unless I have the password to that old account. Help!!!