Consider the analogous question: with all that armour, it seems rather difficult to stop a tank - how can a tank be defeated in battle? The answer is: (a) with your own tanks, preferably more and better, (b) anti-tank weapons, and (c) obstacles.

What are the Medieval "anti-tank" weapons? The most effective personal weapon is the gun (only available in the late Medieval period, but that's when you see the best armour). But even without guns, knights are not invulnerable. This was recently discussed in https://old.reddit.com/r/AskHistorians/comments/9n1blo/how_hard_was_it_actually_to_kill_a_full_plate/

What wasn't discussed there was how to be in a position to use those gun, crossbow, bows, halberds, bills, maces, and warhammers. Numbers, obstacles, and having your own armour will all help. To defeat a knight, use your own knights, preferably more of them. If you don't have knights (or enough knights), armoured infantry with halberds, or shields and maces, or shields and warhammers, etc., can do the job quite well. Again, outnumber the enemy knights. If your infantry are poorly armoured, (a) make the enemy fight dismounted (shooting at their horses can encourage that, so bring lots of archers, or pikemen, etc.), and (b) numbers, discipline, and plenty of halberds or similar.

Fortifications can help a lot, especially against mounted knights. In particular, they can give your gunners, crossbowmen, and archers a lot of time to shoot at the enemy knights at short range.

Without fortifications, if you can't outnumber the enemy knights, or be as well-equipped, don't expect to defeat them. (Thus the popular strategy of retreating to your fortresses.)