I cannot access my backups to delete them and continue using my account

I used dropbox mostly to share files between my iPhone and laptop or PC.  When the suggestion was emailed me to back up all my files in my account, I had thought "Why not?"  I soon found out that my account maxed out, and when my account was frozen for having no more space, there were attempts I made to discard the backed up system files, but I've never been able to get any further use of that account.  My original account was with sbcglobal.net email address.  I eventually linked that with my umich.edu account, and that account doesn't allow alumnus use of the dropbox arrangement the university has.  Since my sbcglobal account is linked to the umich.edu account, I cannot get to my backed up system files to delete them.  I have a terabyte external backup drive for system files, and don't need them in dropbox.  But I cannot use my account at all.  Any suggestions?