How can I switch account log-ins?

Thanks for the quick response! It's for my husband, a laptop with Microsoft Windows. Logging in through the website. I did the same thing for my own login last week, on my own laptop. We're both trying to access the same new Dropbox account. I had an old Dropbox account that I hadn't accessed for a few years. On the login screen, that email address auto-populated. But because I was the one that set that up with my own email, the 2-step verification code was sent to my email and I was able to access that. Once I logged in and then logged out of that old account, I was then able to log into the new account no problem. So, HIS login screen is auto-populating with an old Dropbox account that he did not set up. Trying to do the same thing I did with mine, we've tried to log into that old one so we can logout. But, because someone else created a dummy gmail account to set that up, we have no access to that gmail to receive the 6-digit code. So we're trying to figure out how to permanently delete that old account from his login screen, so he can get access to this new account. Does that make sense?