Backspace will return you to the point you were last typing.
Tap/hold will turn the autocomplete into 'real' text you can then edit - but you can't fill from halfway down the potential URL, it becomes a new full text string that you need to work with, as though you'd already typed it all.
So, for instance, typing
app will turn into
apple.com Tap/hold will 'freeze' that allowing you to then complete
apple.com by typing
/knowledgebase but if you wanted
apple.net you would first have to delete the
com before continuing.