administrator account password isn't working

I had just updated my machine from Mavericks to El Capitan. when it finally finished, I was surprised and worked by the fact that one of the user accounts: the administrator, had reverted to the name it had had before. This computer has two platforms, named primary and secondary. The secondary is the platform left over from when i updated from mountain lion to Mavericks and had, or thought I had to make a new account, then I was too nervous to delete the extra account because I feared something important might be damaged or corrupted. though for a slightly silly reason, I am now glad that i didn't delete it, because it is now the only way by which I am now typing these words. Anyway, when the computer started up, primary had retained the same image above its name, but its name had changed back to the name of a friend of mine who gave me the computer. So, confused and annoyed, but not yet discouraged, I typed in the password with corrects spelling and capitalization. When it didn't work, i tried it in all lower case, and then in all caps. Nothing worked, so i logged in to secondary. It took worrisomely long to load, but eventually did. I was unable to alter anything in the users and groups settings because it requires an admin's name and password. I would appreciate some help in figuring this out, or if nothing can be done, will this same problem occur if I replace the hard drive? it doesn't seem like it would, since, i assume, the user accounts are located on the hard drive.