2008 Honda Accord Changing the oil in my car, question.

I just realized my washer slipped out i thought it put it back in my car, but i was wrong... which is making me know i have to do this again.... i tightened my drain plug already though and no oil is leaking and i drove with it, i don't know im in a rut. my question was, i left my old oil filter in my car, cause i didn't have a wrench to pull it out. but since i already drained my car on oil i just put in new oil. ok anyways straight to the question. Can i change my filter, with oil in my engine already? What happens if i overflow my engine on oil put .5 more quarts than was needed accidently? Is the washer very important if i forgot to install it even though my car isnt leaking?This is the first oil change on my car, and it said oil life is at 5% anyways. I don't know what to do.