On this one website for school you have to put in your username and password to log into any resource. I accidentally typed in my name and password on one line once.. so now every time I type in my username it pops up as a suggestion or whatever.. and I want to delete it.

 

Here's what I'm trying to say in picture form:

 

How can I clear that box?

Delete all saved browser data.

 

On firefox and chrome at least there's a specifc option to "clear saved form data" which will do it, otherwise clearing the entire history should do it.

If you can't do that because it's a school computer, press down on the arrow keys until the one you want to remove is highlighted, then press the delete key.