Editing an attribute and post it in the Mongo collection

If I edit an attribute in a form, what is the best way to update de MongoDB?

More precisely:
Is it a risky practice to update the whole document, or it is better to just detect what attribute is being edited and update only that one?

Of course, filling a form, I will “edit” one attribute after the other one. So, is it better to post in the collection after each attribute editing, or wait an “enter” or Submit button?