How to change my email address when the email is no longer active?

The problematic ones are from people sharing - or at least trying to. I am guessing it is because the email they are sending them too is my newer and preferred email address. However my main Dropbox account is an old one (but with much more storage) and associated with an old address (now changed thanks to you) but of course not changed to my main preferred address as that was not allowed. I changed it to an address I dont really use, its just an alternate for use if needed for any reason. Which means I do have access to it of course.

I dont remember why I opened the other one, perhaps I thought I was changing the email. Not really sure, I am guessing. I dont really want to have two accounts, particularly if this is indeed the cause of the issues. Additionally the newer account has very little space which again I think is the issue as I am getting a message saying I dont have enough space to open one of the documents that has been shared to me.