Umbrella in the Middle Ages are used as a wespon of self-defence. When a wife caught his husband in "treason", which can be a single verbal movement of endorsing Napoleon, the wife can beat the husband to death using the umbrella.
Assuming your force exerted on the umbrella is 100N(Which is enough power to lift a 10kg weight):
When lunging with the umbrella, the contact area of the umbrella is only like about 10cm2 and the pressure exerted with be 1000 Pascals, which is enough to leave a bruise. Especially when umbrellas at that time are reinforced for self-defence.
When downward-slashing with the umbrella, it will be 10000 Pascals(because the contact area of the protruding iron frames of the umbrella are ridiculously small) which is enough to leave a fleshwound when wearing Middle Age clothing(I think).
But now, umbrellas are just shademakers and rain-repellers. You can modify your umbrella into a murdering weapon if you want, but Osu! and I don't take any responsibility of mansluaghter.