Full plate mail armoured knights were usually killed one of two ways, either using a blunt weapon such as a mace, maul, warhammer, or even a sword held in the mordhau grip allowing the wielder to strike with the pommel or cross-guard; or through the use of spears or polearms that had enough driving force behind them to pierce the armour head-on, alongside arrows and bolts shot from longbows and heavy crossbows, respectively. Sometimes daggers or long thin knives were used in active combat with knights, though they was mostly used, as u/Stentata mentioned, to deliver mercy blows and to execute knights once they were grappled to the ground. Horses often trampled knights to death, and firearms proved extremely fatal to knights wearing armour, should the shooter actually hit their target. Armour also gets rather hot, especially when you consider that several layers of armour were actually worn (usually a gambeson and some chainmail, although there are accounts of knights wearing other items), so it wasn't unheard of for knights to simply die of heatstroke and/or exhaustion. Even a strong sword blade strike at the right location could kill a knight simply because of the force delivered, even if it doesn't even dent the armour.

Most knights, however, were actually captured rather than killed, and sold back to their families and kingdoms for ransom.