Almost all medieval armour is open from below at the crotch. So, you can just let go, but you'll wet or foul your underwear. Sometimes there are flaps or removable pieces. Sometimes not. It highly depends on the type or armour and the exact time period.

I have a full maille kit for reenactment. I can easily pee while wearing it. I only have to remove my gloves and helmet (so I can see what I'm doing). Defecating is harder but not impossible, but I just go before gearing up. I don't have armour on all day during reenactment events.